Corresponding author 1 : sara.sedira@univ-biskra.dz 1. Introduction According to ASHRAE standard [1], thermal comfort is defined as "That condition of mind which expresses satisfaction with the thermal environment". For several decades, the influence of environmental parameters on the comfort conditions at the urban scale has motivated a great deal of research. Urban climatology specialists have sought to evaluate the thermal comfort in outdoor spaces, which has acquired its acclaim and becomes of similar importance to interior comfort, widely discussed, where effects of urban configuration on urban thermal climate have also been recently explored through series of investigations in many climatic conditions [2], [3], [4,7]. The level of well-being sensation of the users of outdoors is one of the factors influencing outdoor activities in streets, plazas, playgrounds, urban parks, etc. The amount and intensity of such activities is affected mainly by the level of the discomfort experienced by the users of such spaces when they are exposed to the climatic conditions in these outdoor spaces [8]. Beyond the effect of microclimate on human activity, especially pedestrian, in the spaces between buildings, the spaces outside a building can often influence the perceptions use of a The effect of urban geometry on outdoor thermal comfort. Application of the UTCI index in hot and arid climates. Keywords. urban morphology; urban microclimate; outdoor thermal comfort; hot and arid climate; UTCI index. Abstract. The object of our study is to evaluate the effect of urban geometry on microclimate and outdoor thermal comfort. In this context, “the RayMan pro 2.1” software was adopted to define UTCI index applying in hot and arid climates, exactly, in the urban fabric of the Ksar of The Red Village, El Kantara in Biskra city, Algeria, to define the most efficient urban geometry in term of summer thermal comfort by studying the real impacts of the urban form on the solar control and microclimatic conditions. studying the correlation between the geometry of the street estimated by the ratio H/L, the sky view factor (SVF), its orientation and the evolution of the physical variables (Ta, MRT, Ws) and the values of UTCI index. Significant relationships were found between UTCI index, urban geometry and heat stress in outdoor environments. The results of this research have shown the effect of urban design strategies on modifying the microclimatic conditions in hot summer for outdoor spaces in hot- arid climate. 2 Department of architecture, Oum Bouaghi University, Algeria 23 Technium Vol. 6, pp.23-31 (2023) ISSN: 2668-778X www.techniumscience.com Department of architecture, Biskra University,Algeria. Sara Sedira 1* , Said Mazouz 2 . 1 Laboratory of design and modelling of forms and environments,