Shear failure of RC members is sudden and uncontrolled due to crushing of members into a block of smaller size. The development of critical shear crack cancels shear transfer mechanisms. To minimize failure due to shear the factors which affect shear transfer mechanisms of RC beam causing shear failure should be identified. In many construction sites the box used to measure aggregate is filled beyond the design depth and there is intentional addition of extra volume aggregate for profit in the concrete design mix. Since the major portion of RC beam is occupied by aggregate and cement paste effect of mix design ratio variation on shear capacity needs detail investigation. To evaluate whether the effect of volume increment of aggregate has direct or inverse effect, samples of RC beam without web reinforcement were prepared using C20/25, C25/30, and C32/40 concrete were prepared. The aggregate volume was increased by (5%, 10%, and 15%) for each grade of concrete to prepare beam specimens. The aggregate volume increment satisfies the limit of ACI concrete mix design requirement. The shear capacity of RC beam specimens at 5% aggregate volume increment decreases significantly compared to control mix concrete whereas 10% aggregate volume increment has no noticeable reduction for C20/25 and C25/30 concrete. When aggregate volume was increased by 10% more space in the concrete mix is occupied by aggregate and the bond between concrete and flexural reinforcement become weaker which indirectly increases the dowel shear transfer mechanism of RC beam. For RC beam specimens prepared using C32/40 concrete is different at 10% aggregate volume increment, shear strength reduces considerably below the control mix concrete and 15% aggregate volume increment has smaller reduction compared to 5% and 10% volume increment. At 15% aggregate volume increment shear strength of RC beam specimens prepared using C20/25 and C25/30 concrete decreases back below 10% volume increment even for C20/25 concrete higher reduction was recorded. This experimental result shows for all grades of concrete there is optimum aggregate volume increment beyond the mix design which has relatively negligible reduction in shear strength of RC beam.