Thermal unit indices have a strong correlation with the phenology, growth and yield of crops and can be effectively used to select suitable crop cultivars for specific environmental conditions especially temperature. In this study, four mustard varieties (viz., 'BARI Sharisha-14', 'BARI Sharisha-15', 'BARI Sharisha-16' and 'Tori-7') were grown in two consecutive growing to assess the impact of thermal unit indices on crop growth and development, and to select the suitable variety for better yield under optimum sowing condition. Thermal unit indices viz., growing degree-day (GDD), helio-thermal units (HTU), pheno- thermal index (PTI) and heat use efficiency (HUE) were estimated from daily temperature and sunshine hours. Role of GDD on different growth indicators and seed yield (SY) were estimated through association and dependence of the traits. Significant variations in studied genotypes were observed for different traits. -1 Among the studied varieties, 'BARI Sharisha-16' produced higher dry matter and seed yields (1.82 t ha ) while accumulated maximum GDD at different growth stages. A strong positive association was obtained between GDD and the studied traits. Thermal unit indices had a strong influence in attaining different phenophases and other growth indicators. Therefore, results suggest that those indices could be used for growth prediction; further 'BARI Sharisha-16' is expected to use heat energy more efficiently for increasing the seed yields which indicated that the crop can perform better under global warming scenarios. Keywords : Growth, mustard, phenology, thermal indices, yield Assessing impact of thermal units on growth and development of mustard varieties grown under optimum sown conditions M.R. ISLAM 1 , M.A. ALAM 2 , MOHD. MOSTOFA KAMAL 3 , R. ZAMAN 4 , AKBAR HOSSAIN 5* , ALHARBY, H. 6 , BAMAGOOS, A. 6 , MOHD. FAROOQ 7,8,9 , J. HOSSAIN 10 , C. BARUTCULAR 11 , F. ÇIG 12 and AYMAN EL SABAGH 13* 1 Agronomy Division, Regional Agricultural Research Station (RARS), Bangladesh Agricultural Research Institute (BARI), Ishwardi, Pabna, Bangladesh; 2 Plant Breeding Division, RARS, BARI, Ishwardi, Pabna, Bangladesh; 3 Research Fellow, Centre for Crop Health, University of Southern Queensland, Australia; 4 Agricultural engineering Division, RARS, BARI, Ishwardi, Pabna, Bangladesh; 5 Bangladesh Wheat and Maize Research Institute, Dinajpur-5200, Bangladesh; 6 Department of Biological Sciences, Faculty of Science, King Abdulaziz University 21589 Jeddah, Saudi Arabia 7 Department of Crop Sciences, College of Agricultural and Marine Sciences, Sultan Qaboos University, Al-Khoud-123, Oman 8 The UWA Institute of Agriculture and School of Agriculture & Environment, The University of Western Australia, Perth WA 6001, Australia 9 Department of Agronomy, University of Agriculture, Faisalabad-38040, Pakistan; 10 Pulses Research Centre, BARI, Ishwardi, Pabna, Bangladesh; 11 Department of Field Crops, Faculty of Agriculture, Cukurova University, Turkey 12 Department of Field Crops, Faculty of Agriculture, Siirt University, Turkey; 13 Department of Agronomy, Faculty of Agriculture, Kafrelsheikh University, 33156 Kafrelsheikh, Egypt; *Corresponding authors: ayman.elsabagh@agr.kgr.kfs.edu.eg, tanjimar2003@yahoo.com ABSTRACT Mustard is one of the major winter oilseed crops grown across the globe. While it requires cool and dry weather for optimal growth and development. It is sensitive to changes in weather and hence climate change could have a significant impact on its production. In existing practice, a greater part of the vegetative phase is completed when the temperature is relatively high, but during flowering, the temperature is low and later on as the crop reaches maturity, the temperature and photoperiod gradually increase (Nanda 1995). et al., Prevailing weather conditions during the whole crop growing season have a direct association with the phenological developments in the crop plants, which modulate yield formation (Akhter 2016). The morpho-physiological et al., developments are markedly influenced by air temperature during different phenological stages (Srivastava 2005); et al., more importantly, the sensitivity of crop plants increases in Journal of Agrometeorology 21 (3) : 270-281 (September 2019)