This research study investigated the onset of menarche among adolescent school girls living in non-gas and gas flaring environment in Bayelsa state. The participant in this study were eight hundred and twenty six (826) in number. The result obtained from the study shows that one hundred and fifty six (156) within the ages of 10-14years making (18.9%) are yet to attained menarche from gas flaring environment while (81.1%) have attained menarche in both environment. The overall age at menarche compared with body mass index was (12.6yrs) and (17.3kg/m²) from both group. Furthermore the age at menarche among adolescent residents in non-gas flaring environment was (11.8yrs) compared with those in gas flaring environment (12.9yrs). However the mean age at menarche among adolescent girls attending private schools was (12.7yrs) with a body mass index of (18.5kg/m²) in comparison with (12.5yrs) and (16.7kg/m²) of adolescent school girls attending public schools. The result from this study has shown that (50.8%) of school girls from gas flaring environment have attained menarche compared with (30.3%) from non-gas flaring environment at the age of (11.8yrs) with an increase positive percentage difference of (34.1%) between body mass index and the age at menarche among non-gas flaring adolescent and (37.2%), (22.9%) in gas flares stack surrounded communities in Bayelsa state.