Birth weight is one of the best indexes for determine mother’s nutritional status and pregnancy care. The main aim of this study is determine the association between mother’s BMI and birth weight based on age in rural areas in the north of Iran. This study was established on the 2459 under-five- children that were chosen from 20 villages. Birth weight and Birth height have been recorded by health file in primary health system and mothers BMI has been measured. SPSS 18.0 software was used for statistical data analysis. T-test and ANOVA were used for comparing quantities groups. P-value under 0.05 included significations. Totally, the mean ± standard deviation of birth weight and birth height was 3211.9±520 g and 49.7±2.75 cm, respectively. Statistical differences were significant in both of them among three age groups (P=0.001 for all).Post hoc test was significant between age groups (except between 25-34 years and > 35 years age mothers based on birth weight)(P<0.001 for all). In >25 years mothers, the association between mother’s BMI and birth weight was not steady, while it was increased with mother’s BMI in 25-34 years (P=0.001) and in >35 years age groups(P=0.001). The mean of birth weight and birth height increased when mother’s BMI increased except for birth weight in <25 years mothers. Dependence was found between mother’s BMI and birth weight and birth height in older mothers while it was not established in younger of them. Needing food supplement in younger pregnant women should be considered in future.