Knowledge on fertility behavior of thermo-sensitive genic male sterile (TGMS) line is highly essential for increasing hybrid rice production. A set of four promising TGMS lines viz., GD 98014, GD 99017, GD 98029 and GD 98049 was characterized for their fertility behaviour under field conditions. The pollen and spikelet fertility data revealed that all lines had stable sterile phase with 100 per cent pollen sterility for more than 50 consecutive days during high temperature condition (30/20 ⁰C maximum/minimum temperature) and they reverted to 60% pollen and spikelet fertility during low temperature condition (less than 30/20 ⁰C). The critical panicle developmental stages sensitive to temperature were ascertained for each line. The daily mean temperature, relative humidity and photoperiod appeared to influence fertility alteration in some lines. The environmental conditions influencing fertility alteration varied among the lines. However, all the lines satisfied the requirement of stable fertility behaviour to the level of commercial exploitation in two-line hybrid breeding and found as potential TGMS donors to develop new TGMS lines.