Background and Objective: Children suffering from failure to thrive (FTT) may have significant impairments in physical growth as well as cognitional and developmental functions .After 6 months of age approximately one third of the energy needed for a child should be provided by supplemental nourishments. Studies have shown that poverty itself is not a factor of malnutrition alone and the role of the mother’s knowledge in proper use of the family income and the use of the supplementary nutrition in appropriate time with proper food is significant. The aim of this study was to evaluate maternal knowledge about nutritional status of children and its related factors. Materials and Methods: This cross sectional study was conducted on three hundred mothers who had a child between 6 to 12 months, at the health care center of the community oriented medical education in Zanjan University of Medical Sciences .The mothers were interviewed about supplemental nutrition. Information about maternal age, education, number of children, and family income were also gathered. Results: One hundred eighty one (60/3%) of mothers were between the ages 20-29years and 35/3% had only elementary education. Fifty five percent lived in rental houses and 47/5% had one child. 42/3% of them were found to have average knowledge. Eighty two percent knew the appropriate time to start supplemental nutrition but 92% did not know that the best time of supplemental nutrition was after breast feeding. Conclusion: According to this study, there was a significant relationship between maternal knowledge about supplemental nutrition and maternal age, education, and the number of children P (0.0001). However there was no significant relationship between mother’s knowledge and family type of home ownership. It seems that up-grading maternal knowledge plays an important role in prevention of FTT.