This study was carried out during two successive seasons of 2005/2006 and 2006/2007 to study the effect of application time and rate of foliar fertilization with complete fertilizer  solution containing 20%, 20%, 20% of N, P and K at the rates of 2, 4 and 6 g L^-1 on vegetative growth, flowering and some chemical constituents of Brassica nigra [L.] Koch. All foliar fertilization treatments exhibited remarkable significant effects on plant height, number of branches, number of leaves, number of inflorescences, herbage fresh and dry weights, inflorescence fresh and dry weights and seed yield dry weight when compared with untreated plants. Chemical analysis showed that the highest content of N, P, K were obtained, in general, from plants sprayed with 4 g L^-1 of the complete fertilizer solution sprayed either every 10 or 20 days. Concerning the micronutrient contents (Fe, Mn, and Zn) in leaves of B. nigra [L.] Koch, foliar fertilization resulted in significant effect on Fe, Mn and Zn in leaves when compared with untreated plants. The highest content of chlorophyll a and b in leaves was found in plants sprayed with complete fertilizer solution at the rate of 4 g L^-1. Generally, the maximum increase in the studied parameters resulted from spraying plants with 4 g L^-1 of complete fertilizer every either 10 or 20 days. So, economically the results recommend spraying mustard plants with 4g L^-1 complete fertilizer 20-20-20 every 20 days.