In 2011 and 2012 seasons, two field experiments were conducted at Sahl El-Tina (North Sinai Governorate, Egypt) to evaluate the effect of partial substitution of 33.3 % and 50% of the N rate recommended by the Ministry of Agriculture (80 kg N ha' i.e.34 kg N fad."') using biofertilization with Rhiobium radiobacter sp strain on cowpea (Vigna unguiculate L. c.v. Kafr El-Sheikhl).This study represents a new strategy for understanding the most efficient use of N-bio source, whether be under demand for agricultural utilization. In addition, it overcome the possible adverse fears of human health through environmental risks as a result of excessive use of the nitrogenous fertilizers as well as support the newly technique of "bio-organic" agriculture. Seeds of cowpea were sown under mineral-N fertilizer (as ammonium nitrate) and compared with biofertilization or biofertilization with mineral-N. Yield and yield components as well as macro and micronutrients contents of cowpea pods and seeds were increased as a result of applied different Bio and/ or N-rates and their combinations. Seed protein content and protein yield were increased significantly and followed the order of Biofertilization + 50% of nitrogen recommended dose (N-RD) >100% N-RD >Biofertilization + 33.3 % N-RD > Biofertilization. Fertilized treatments decreased values of soil EC and pH and increased soil available N, P and K as well as Fe, Mn, Zn and Cu content after harvest. The treatment of Biofertilization + 50% N-RD was superior to the other treatments.