The use of natural fiber bolster polyurethane composite foams is changing into more and more fashionable within the cellular foam industry. This demand is due to the increasing use of environmentally friendly, biodegradable and sustainable materials to manufacture polyurethane composites for such applications. In this regard, natural fibers from agricultural wastes are preferred over their synthetic counterparts because of their ready availability, light weight, biodegradability, strength and cost effectiveness. In addition to the renewable nature, the use of natural fibers in polyurethane foams leads to composite foams with better properties than pure polyurethane foams (PUFs). It was made by foaming a liquid mixture of isocyanato-polyols in presence of a blowing agent. This article contains an experimental study of improving the reinforcement of rigid polyurethane foams (RPUFs) by the means of incorporating various amounts of each cotton stalks (TS) and corn stalks (CS). Five different percent volume of reinforcement (5, 10, 15, 20 and 25) of cotton or corn stalks were used with (RPUFs), beside the control ratio of (0 %). The influence of incorporating (TS) and (CS) on rigid polyurethane foams (RPUFs) is reported. A produced composite which mad from “RPUFs" and “TS and CS" were prepared for testing to measure bulk density “Bd, kg/m3", water absorption “Wa, %", compressive strength “P, MPa" and deformation “D, mm". The results showed that the previous physical and mechanical properties were affected by previous percent volume of reinforcement. It indicated that there were some improvements on some physical and mechanical properties of produced composite. There was a water absorption reduction ratio of 82.31 and 42.97 % at reinforcement ratio of 25 % for cotton and corn stalks respectively, in comparison with control (0%). Also, there was a compressive strength increasing ratio of 94.40 and 82.76 % at reinforcement ratio of 25 % for cotton and corn stalks respectively, in comparison with control (0%). Cotton stalks showed the best, lowest, water absorption values than corn stalks and it showed the best, highest, compressive strength value than corn stalks. The lowest value of water absorption was 51.79 % at reinforcement ratio of 25 % and the highest value of compressive strength was 0.89 MPa at reinforcement ratio of 25 % for cotton stalk.