Changes in the seasonal population of Thrips tabaci lind. attacking onion crop and the effect of certain weather factors were studied throughout (2015/2016 & 2016/2017) onion growing seasons. T. tabaci appeared in few numbers by the 1st week of January. Dynamics of the nymph and adult stages showed increasing steadily 3th week of February until 1st week of April. The values of correlation coefficient infer that infestation and degree days are positive correlated and non- significant. The weather factors as maximum temperatures and maximum & minimum relative humidity didn't give significant correlation. On the contrary, the minimum temperature give significant correlation where r= 0.509 with probability p= 0.031 during first season. On the other hand, all weather factors were positively correlated with non significant except the minimum R.H. during second season. Considering, plant age as polynomial relation (y=a+b1x +b2x2+b3x3) resulted in strong and significant correlation and regression.