Abstract: This study especially focuses on the indirect method of estimating undocumented migration to the state of Assam in Northeast India in absence of adequate and accurate migration data. In this method the census base population of age and sex has been projected through the Leslie matrix population projection model by using estimated life table survival rates and age specific fertility rates of Sample Registration System of India. The study also investigated the affect of illegal migration in the growth of different religious and language speaking population in all districts of Assam. The result showed that population growth is unusually high in those districts of Assam bordering Bangladesh. This growth especially shows remarkable among religious groups of Muslims and among linguistic groups of Bengali speaking population. This is very much suggestive of cross border migration from Bangladesh. The volume of undocumented migration in Assam during the period 1971-2001 is being estimated by using Matrix method of population projection and is turned out to be 830755 and 534819, respectively during 1971-1991 and 1991-2001.