This paper examines the significance of nano silica and micro silica mixture on the shear strength of reinforced concrete beams. The nano and micro silica mixture was used as a partial replacement for Portland cement with different proportions to produce four (4) concrete mixtures. In this research, the tests on fresh concrete mixtures (slump test, density test and air content test) were performed. The compressive strength of concrete mixtures was tested. The shear strength of reinforced concrete beams with and without shear reinforcement is examined. Moreover, the influence of the multi-sized silica mixture on other mechanical characteristics of concrete related to the shear behavior of concrete beams, shear crack width, the inclination angle of shear cracks and deflection were also examined. From test results, the mixture of 2% nano silica with 8% micro silica was found to lead to the highest compressive strength, it improve the concrete compressive strength up to 33% compared with control specimens. The shear strength of the concrete beam incorporating 8% micro silica and 2% nano silica mixture was also improved, it increased by 30% and 59 % for beams with and without steel stirrups, respectively. The significance of nano-micro silica mixture on the concrete contribution to the shear strength of reinforced concrete beams is examined. The nano-micro silica mixture increased concrete contribution to shear strength of reinforced concrete beams up to 17%.