Radon, thoron and their progenies as a natural radiation hazards to human health is well known. These gases are present in the environment and their level of concentration depends upon geographical and geological conditions, meteorological factors, etc. The indoor radon/thoron concentration is also influenced by building materials, ventilating system and soil gas diffusion. Measurement of radon/thoron concentration in Mizoram is reported in this paper covering three districts, namely Aizawl, Kolasib and Champhai. In this study, we used solid-state nuclear track detectors to obtain the time integrated concentration levels of indoor radon/thoron. The study was conducted by measuring the cumulative exposure for a period of about 90 days each in 149 houses during rainy season (May-August). Houses were selected on the basis of geological characteristics of the area and the construction types in order to determine variation of concentrations of radon and thoron due to these factors. Among the three districts, Champhai District had the highest radon/thoron concentrations, while Kolasib District had the maximum thoron concentration. Among the different types of houses, concrete building had the average maximum concentration of radon followed by Assam type building with G.I. Sheet walls while the contribution due to asbestos walls of Assam type building was found to be lowest.