Objectives: The event of menarche is an exceptional phenomenon, which occurs once in a lifetime. It is the precursor of future fertility. The onset of menarche is mainly dependent on genetic factors, however geographical and nutritional factors among other factors have been attributed to the events of menarche. The aim of this study is to determine the age of onset of menarche and the factors which induce it in northern Iran. Methods: This is a cross sectional study which will assess the onset of menarche in girls between the age of 11-16 years. The subjects were either in middle or high school and had experienced menarche within 6 months prior to the study. This study revolves around 600 girls who were selected using stratified randomized sampling techniques. The data was collected in the form of a questionnaire which focused on age, weight (was measured to the nearest 0.1 kg using a balance beam scale), height (was measured to nearest 0.1cm with a manual height board), and Body Mass Index (BMI: kg/m2) was used as an index of relative weight. Principal Component Analysis (PCA) was applied to analyze Socio-economic Status (SES). The defined standard age of the first menstrual event was applied as the standard. The data collected was analyzed using the Statistical Package for the Social Sciences (SPSS.10) statistical analysis software and the Chi-square and Analysis of Variance (ANOVA) tests were applied. In an effort to establish the factors associated with the age of menarche, multivariate analysis was performed based on linear logistic regression which was performed using a model where all the variables changing the risk estimated by more than 10%. Statistical significance was determined at P<0.05. Results: The mean age at menarche was observed to be 12.99±1.33yrs (ranging from 10.16 to 15.91 yrs). The study showed that menarche occurred during the summer for 270 (45%) patients making it the most common season for menarche while winter was the least common season. The mean age of menarche was delayed in patients from low SES groups and it occurred earlier in patients from high SES. In girls with low BMI, menarche occurred earlier in comparison to girls with high BMI. There was a significant correlation between session and age of menarche (p<0.05). There was no significant relationship statistically between BMI and age of menarche (p>0.05). Linear regressions showed no significant effect of BMI on the age of menarche but there was a significant correlation between season and SES on the age of menarche. Conclusion: The age of menarche and the most common seasons for menarche were similar to previous studies. Results from this study showed that the age of menarche decreased with higher SES, and also the age of menarche in girls with high and low BMI occurred earlier. However, previous studies showed that it ocurred earlier in high BMI and later in low BMI. It appears that genetic and environmental factors have influenced these differences.