Objective: Cerebral palsy Spastic Diplegia (CPSD) is the most common motor disability in childhood. It is a neurodevelopmental condition beginning early in life and continues throughout life. In addition to mobility problems, the environment can also affect the CPSD children’s social participation. This study aims to investigate the relationship of environmental factors with the social participation of children with CPSD. Materials & Methods: This cross-sectional study was conducted on 116 parents of children with CPSD referred to the occupational therapy clinics in Tehran, Mazandaran, and Alborz provinces of Iran in 2018-2019. They were recruited using a convenience sampling method. Parents completed the European Child Environment Questionnaire (ECEQ) and the life habit questionnaire to assess their perception of the environmental factors and their children’s participation, respectively. The Gross Motor Function Classification System-Expanded and Revised (GMFCS-E&R) was initially used to classify the gross motor function (walking ability) of the children with cerebral palsy living in Europe (The SPARCLE project). We used it to estimate cognitive levels in children. The Pearson correlation coefficient and linear regression of SPSS version 22 were used to analyze the results. Results: Based on the GMFCS-E&R classification, 23.3% of children were at level I, 27.6% at level II, 31.9% at level III, and 17.2% at level IV. Moreover, 89.7% of the children had an IQ>75, and 85.3% had no seizures. All domains and sub-domains of ECEQ had significantly correlated with social participation (P<0.01), where the physical environment had the highest correlation (r=-0.811, P<0.01). Based on the linear regression model, physical environment (β=-0.475, P<0.01) and attitudinal environment (β=-0.285, P<0.05) were the predictors of social participation (adjusted R2=0.71). Conclusion: Physical and attitudinal environments are predictors of social participation in children with CPSD. Environmental modifications and adaptations at home, community, school, and especially transportation system can increase the social participation of these children.