Background & Objectives: Mental retardation is a critical deficiency in cognitive functions, social skills, and adjustment behaviors. About 1 to 2 percent of people are mentally retarded because of various factors. The purpose of this research was to study the prevalence of mental retardation in Ardebil city and determining the role of environmental and Heredity factors on mental retardation. In other words, which of pre-, on-, and post-birth factors as well as environmental factors are related to mental retardation of 7-11 aged children.

 Methods: In order to find out the prevalence of mental retardation, a sample of 500 families were selected by simple random sampling method and were studied by causal-comparative method. Also percentage, frequency, and comparing of two independent groups were used to analyze the data and study the extent of prevalence.

 Results:�The results showed that the rate of prevalence of mental retardation is 3.6%, which is higher than the extent of prevalence of the world. Also the results of comparing two groups normal and mentally retarded revealed that familial marriages, in-birth events, fever and convulsion diseases during post-birth period are higher in mentally retarded group. Furthermore, the results showed that the level of mothers’ educations in normal group is significantly higher than that of mentally retarded group. And among the other factors, environmental factors were the most important factors of mental retardation of children in Ardabil city.

 Conclusion:�The high rate of prevalence of mental retardation and the role of environmental factors in mental retardation of children in Ardabil city require environmental intervention such as training of families and improving health level of society.