This paper presents an experimental investigation on the effect of Nano Silica (NS) on the compressive strength of concrete made with different cement contents. Two percentages of NS, 1.5% and 3%, as partial replacement of cement were investigated at cement contents of 300, 400, 500 and 600 kg/m³. The effect of NS and cement contents on the compressive strength of concrete at different testing ages of 7, 28 and 56 days was evaluated. The effect of NS and cement contents on the water absorption percentage (WA%) was also investigated. The effect of cement content on the compressive strength ratios at 7 and 56 days to that at 28 days was found and correlated. The effects of cement content on the enhancement ratios of compressive strength and WA% were also evaluated and correlated. NS addition enhanced the compressive strength and reduced the WA%. The maximum enhancement in the compressive strength was recorded at 1.5 % NS. As this ratio increased to 3%, the compressive strength decreased but to a values not less than that of the concrete mix without NS.