In Urmia city, many children learn and speak their first language (either Azari or Kurdish) at home and study all of their courses in Farsi throughout their education. This bilingual quality of education needs to be researched to attain high quality educational practices. The purpose of this study was to examine the effects of bilingualism on cognitive development. 135 primary bilingual and monolingual students in grades 1, 3, and 5 were tested. The data were collected through tests that rely on Piaget's theories. The major finding showed that increase in age predicted the cognitive development stages of children as hypothesized. It was also found that there are no signification differences between bilingual and monolingual groups on Piagetian tasks. (Contains 5 tables.)