An experiment was conducted at the Horticulture Farm, Bangladesh Agricultural University (BAU), Mymensingh during the period from October, 2002 to April, 2003 to study the effects of planting date and concentration of GA3 on the growth, yield and quality of onion seeds. There were two planting dates viz., 30 October and 20 November and five concentrations of GA3 viz., 0, 25, 50, 75 and 100 ppm. The two-factor experiment was laid out in the split plot design with three replications. The planting date and concentration of GA3 showed significant influence on the growth of plant and yield of onion seed. The highest yield of 392.27 kg/ha was obtained from 30 October planting. The maximum seed yield (389.67 kg) was found at the concentration of 75 ppm GA3 and the lowest seed yield 379.60 kg per hectare was noticed from the control plant. The treatment combination of 30 October planting x 75 ppm GA3 gave the highest seed yield (398.67 kg/ha). The quality of onion seeds varied significantly due to planting time and concentration of GA3 used. The highest weight of thousand seeds (3.15 g) was obtained from 20 November planting time and 75 ppm GA3. The highest germination percentage (78.90)was recorded from the treatment combination of 30 October planting x 75 ppm GA3.