Static loading tests were conducted on 16 steel-concrete composite beams and two steel beams to investigate shear resisting mechanisms and the strength of composite beams. The main experimental parameters were the shear span aspect ratio of the simply supported beams, and the width and thickness of the concrete flanges. Based on strain measurements, stress in the steel beam was analyzed using theories of elasticity and plasticity, and the vertical shear that the steel beam resisted was calculated. The shear resistance of the concrete flange was then obtained by subtracting the steel shear contribution from the total load applied. It was found that the concrete flange could sustain 33-56% of the total ultimate shear applied to the composite beam specimens, contrary to the typical assumption of neglecting the concrete shear contribution in most design codes and specifications. A shear strength equation that considers the shear contributions of both the steel beam and the concrete flange is proposed.