49 International Journal of Child Development and Mental Health CDMH Abstract The purpose of this study was to develop and investigate the effects of equine assisted therapy on social and undesirable behaviors of youths with autistic spectrum disorder (ASD). Six youths with autistic spectrum disorder aged 14-25 years were recruited by purposive sampling. They attended the 8 session program, twice a week. Each session took 80-90 minutes. The program included 4 activities: orientation/introduction, food preparation and feeding, grooming, and leading a horse. Subjects’ social and undesirable behaviors were assessed before and after the program using the following: 1) Recording forms for social behaviors and undesirable behaviors (Sung-U et.al, 2009) 2) The Scale of Independent Behavior-Revised (SIB-R) (Bruininks, 1996) using subtests for social interaction and communication skills, and a subtest for undesirable behaviors: frequency and severity of these behaviors. The Wilcoxon Signed-Rank test was used for data analysis, to compare units of measurement before and after the program. The results showed scores of SIB-R in subtests of language comprehension and language expression. The total scores were signifcantly increased (p<0.05). However, the subtest of social interaction was not signifcantly increased (P>0.05). Furthermore, the frequency scores of undesirable behaviors were not signifcantly decreased (P>0.05), but the severity score of these behaviors was signifcantly decreased (p<0.05). The social behavior score from the record forms was not signifcantly increased (P>0.05), but scores of undesirable behaviors were signifcantly decreased (p<0.05). The study concluded that a program of equine assisted therapy can encourage social behaviors and reduce undesirable behaviors in youth with autistic spectrum disorder. Keywords : Youth with autistic spectrum disorder, Program of equine assisted therapy, Social behaviors, Undesirable behaviors. Introduction Autistic Spectrum Disorder (ASD) is a neurological developmental disorder. People with ASD have developmental impairment in social and communication skills. They also have repetitive behavioral patterns and rigid interest that lead to limitations in their lives (Siriratraka, 2012). Animal Assisted Therapy (AAT), which involves an appropriate animal as part of a therapeutic program, is regarded as one, among various kinds of therapeutic approaches that can help ASD clients. Equine assisted therapy, which uses horses, is one AAT taking advantage of the movement of horses to improve physical, social interaction, behavioral, psychosocial, and cognitive functions of the clients (Latella & Langford, 2008). Impairment in social interaction and behaviors are typically seen in ASD persons as they experience diffculty in starting interaction or communication with others, as well as shaping their appearance and behaviors properly into contexts (Siriratraka, 2012). According to advantages of the equine assisted therapy Effects of Equine Assisted Therapy on Social and Undesirable Behaviors of Youths with Autistic Spectrum Disorder Sineenart Boonmee 1 , Maethisa Pongsaksri 1 , Siriporn Peansukmanee 2 1 Department of Occupational Therapy, Faculty of Associated Medical Sciences, Chiang Mai University 2 Department of Companion Animals and Wildlife Clinic, Faculty of Veterinary Medicine, Chiang Mai University E-mail: Sinee161fai@gmail.com