Two field experiments were conducted at South of Ras El-Hekma (56 km east of Marsa Matrouh city) under rainfed conditions during 2005/ 2006 and 2006/ 2007 winter seasons, to show the response of pure stands of barley (Hordeum vulgare L.), vetch (Vicia sativa L.) and their mixture (75 % barley and 25 % vetch) to P-fertilizer levels (0, 7.5, 15 and 22.5 Kg P₂ O₅/ fad) to study the effect of cropping systems and P-fertilization on grain and straw yields, as well as, field stubble and water use efficiency.  The obtained results scribed that, barley- vetch mixture (75 % barley + 25 % vetch) at different growth stages; i.e., booting, 50% heading and physiological stages, was superior to mono-cropping system of barley and vetch in fresh forage, dry matter and field stubble yields, as well as, crude protein yield in both seasons. Also, at harvesting time; i.e., full maturity stage, B-V mixture gave the highest grain and straw yields with higher total crude protein yield. Mixed cropping system was efficiently in water-use compared to pure stands of barley and vetch. The land equivalent ratio (LER) was more than one for mixed cropping system, which also, led to the highest LER in respect to dry matter yield, crude protein yield at different growth stages and harvesting at full maturity. Applying of higher P-fertilizer level; i.e., 22.5 Kg P₂O₅/ fad gave the highest of studied traits. Economically, barley-vetch mixture with P-fertilizer level of 22.5 Kg P₂O₅/ fad gave the highest values of gross and net returns, while, B-V mixture with application of 15 Kg P₂O₅/ fad achieved the highest values of average rate of return (ARR). Ultimately, these results ascertained the importance of entering some of contingent forage crops, such as vetch to associate with barley in mixed-cropping system particularly, with P-fertilization, also to improve the crop/ livestock farming system, which might be contribute in sustainable development of rainfed conditions at Ras El-Hekma, Matrouh, Northwestern coast of Egypt.