The fertility of a population is always influenced by various factors, directly or indirectly and these factors are, however, not the same for every population. The present study is an attempt to validate the various biological and social factors including son preference that influence the fertility among the three endogamous tribes of Arunachal Pradesh – Galo, Adi and Hills Miri. In together 624 ever married women were interviewed from both rural and urban areas using pre-tested schedule. It is found that one fourth of the male and one third of the female are marrying before the legal age at marriage among the study populations. It is higher among the rural compared to urban irrespective of gender and so the mean live birth among the rural are higher than the urban for all the study tribes. The age at marriage and age at first conception has inverse relation with the fertility. The mean live birth is higher among the illiterate mothers and house wife compared to literate and working mothers, respectively. The birth control users have a lower mean fertility compared to nonusers. Preference for son is observed among Galo and Hills Miri and gives an impact on fertility. Mother with lesser number of son have higher mean live birth.