Flowering and fruit set of sweet pepper (Capsicum annum L.) is sensitive to environmental condition and nitrogen (N) nutrition status. To clarify the N fertilization effect, four levels of total N concentrations and four nitrate-nitrogen (NO3-N) and ammonium-nitrogen (NH4-N) ratios were supplied in three distinct physiological stages of sweet pepper: Stage I - vegetative; Stage II - fruit set during cross-pollination; and Stage III - fruit developing period. The plants were grown hydroponically during two seasons. In the season of short photoperiod progressing from autumn to winter with 10.9 to 10.2 light h day-1, gradually increasing the total N concentration with the progressing physiological stages from 3 to 6 and 9 mM increased the total set of flowers and fruits, and produced the highest total fruit yield (3444 g plant-1). During the increasing day length season from spring to summer (12.3-12.8 h day-1), supply of 9 mM N and 30% NH4-N before fruit set produced the larger amount of early ripening fruits. When the NH4-N percent rose from 0 to 15 and 30 in the autumn-winter season, the highest amount of early ripening fruits was obtained. In both seasons, the highest total fruit yields was obtained when NH4-N was 30% of the total N (6 mM) during the vegetative stage and NO3-N as sole N source during fruit filling stage.