Effect of three planting dates of onion plants (November, 15^th, December, 15^th, and January, 15^th) on levels of infestation by thrips, Thrips tabaci Lindeman and on its yield, were studied during two successive seasons (2013/14 and 2014/15) at El-Beheira Governorate, Egypt. The degree of infestation with thrips increased significantly by delaying planting date, as onion plants cultivated in the earliest planting date (November, 15^th) were attacked by the fewest numbers with highest weight of onion bulbs, while the plants of the latest planting date (January, 15^th) were more liable to insects infestation with lowest weight of onion bulbs. The population density and life stage composition of T. tabaci populations were determined in the two consecutive seasons and in the three planting dates. The intra-plant distribution of thrips was studied and a clear pattern in the diel periodicity of the intra-plant distribution of T. tabaci, was noticed in that the adults tended to be located higher on the plant during the hottest periods of the afternoon. The vast majority of nymphs occupied the basal half of the leaves and remained there throughout the day. The effect of thrips damage on onion yield was studied and the mean percentages of yield losses in onion bulb as a result of thrips damage during two successive seasons ranging from 26.17 % to 42.39 % in different planting dates in the 1^st season. In the 2^nd season loss in bulb yield recorded 20.45, 30.85 and 35.23 % in the 1^st, 2^nd and 3^rd planting date, respectively. Statistical analysis of correlation coefficient values showed that the effect of climatic factors on the population density of T. tabaci wasn't differed according to planting date during two studied seasons. The population density of onion thrips correlated positively with minimum, mean, maximum temperature, mean of wind speed and negatively correlated with average relative humidity.