The present investigation was conducted dur-ing two successive seasons to study the effect of replacing mineral nitrogen fertilization by organic nitrogen source with or without biofertilizer addition on Washington Navel orange trees, under sandy soil conditions. Percentages of 100, 50, and 25% of mineral nitrogen were replaced by 50, 75 and 100% of organic nitrogen with or with out Saccha-romyces cerevisae (yeast) as a source of bioferti-lizer. Treatment of 50% mineral N + 50% organic N and treatment of 100% organic N as well as the control (100% mineral nitrogen) recorded the high-er values of N in the leaves especially in the sec-ond season. Treatments included organic fertiliza-tion raised N% in the second season. Phosphorus percentage in the leaves did not differ by treat-ments in both seasons. All treatments enhanced K content in the leaves especially in the second sea-son. On the other hand, adding biofertilizer treat-ments improved N, P and K content in the leaves especially in the presence of mineral N. As for yield per tree, all treatments increased number of fruits and yield weight/tree than the control especially in the second season. The effect of treatments on fruit quality was clearly shown in the first season, since most of the treatments improved fruit quality parameters especially treatment of 50% mineral N + 50% organic N fertilizers. On the other side, treatments containing organic or bio-fertilizer re-duced nitrogen, nitrate and nitrite values in fruit juice comparing with the control (100% mineral N fertilizer).