The basic purpose of this research work is to find out the effect of demographic factors on fertility behavior of people in western Terai of Nepal. Trend in fertility is analyzed statistically in detail not only district wise but also for the whole of the western Terai of Nepal on the basis of primary data collect by the researcher. Among different variables, affecting fertility wives’ education is found as the most effective variable. It has also been observed that an upward shift in the occupational status, affects the fertility behaviours of couples negatively. Similarly there exists an inverse relationship between income and fertility. Among the couples of different religions Muslims couples have the highest fertility and Hindus have the lowest. Rural respondents have the higher fertility than the urban ones. Among the various castes analyzed it has been deduced that Janajati couples have the lowest mean fertility in the study area. Female respondents use contraceptives more than the male ones.Key words: Demographic factors; Fertility; Surviving' Habitat; KapilvastuEconomic Journal of Development IssuesVol. 11 & 12 No. 1-2 (2010) Combined IssuePage: 99- 111Uploaded date: 10 April, 2012