Changes in the seasonal population of Thrips tabaci lind. attacking onion crop and the effect of environmental factors were studied throughout (2006/2007&2007/2008) onion growing seasons .it appeared in few numbers by the 2^nd week of February . Dynamics of the nymphal stage showed increase steadily after half of February until end of March , and highest decrease in population density of thrips was recorded at temperature 24.52 and relative humidity 69 in  thrips populations. The values of correlation coefficient infer that infestation and degree days are highly correlated . The simple regression value (b) indicate that for every  1.0 temperature increase in degree days there are 3.5 individual increased and so on. In onion growing season after rain falls population of T. tabaci decreased.