A field experiment was conducted to study the effects of boron (B), molybdenum (Mo), zinc (Zn) and their interactions on seed yield and yield formation of rapeseed (Brassiea napus L. war. Huashuang 4). Application of B fertilizer to a sandy soil increased the seed yield by 46.1% compared to the control and also created a considerably higher seed yield than the two treatments solely applying Mo and Zn fertilizers, which suggested that B was a main constraint for the seed yield of Huashuang 4 in this experiment. The effect of B fertilizer on the seed yield was attributed to an increase in the number of seeds per silique and siliques per plant. The combined application of B with Mo or Zn resulted in higher seed yield than the application of B, Mo or Zn alone, and the seed yield of the B+Mo+Zn treatment was the highest in all treatments, 68.1% above the control. Dry matter accumulation of seed followed a typical S-shaped curve and it was higher in plants supplied with B than in plants without B. A small but significant increase in the seed oil content and an improvement in the oil quality were also observed in all treatments compared with the control. These results suggested that optimal micronutrient application could provide both yield and quality advantages for rapeseed in poor soil.