In present work, we studied the effect of substitution of silica nanoparticles (SNPs), by replacement of cement on ultrasound pulse velocity and compressive strength of concrete specimens. We also obtained correlation between ultrasound pulse velocity (UPV) and the compressive strength. The mean particle size of silica nano-particle was 20nm. The quality of concrete specimen was assessed by measuring ultrasound pulse velocity (UPV) in m/s and compressive strength (N/mm2). The average value of UPV on 7th day of curing turned out to become 3200 ± 36, 3215 ± 42, 3290 ± 41, 3349 ± 24, 3450±17 and 3456 ± 12 for 0%, 0.5%, 1.0%, 1.5%, 2.0% and 2.5% content of SNPs in the specimens respectively. Similarly, the average value of UPV on 28th day was 3540 ± 36, 3580 ± 38, 3696 ± 42, 3820 ± 39, 4160 ± 40, 4163 ± 41 for same amount of substitution of SNPs respectively. It had been observed that the UPV was higher in the specimens replaced by silica nano-particles (by weight of cement) and it achieved maximum strength at nearly 2% (that is in between 2.0-2.5%). The average compressive strength on 7th day was 25, 25, 27.6, 30, 32.4 and 32 N/mm2, but, on 28th day the it increased up to 38, 38.5, 40, 42, 48.5 and 48.8 N/mm2 for the same content of silica nanoparticles ( 0%, 0.5%, 1.0%, 1.5%, 2.0% and 2.5%) respectively. As UPV increased so did the compressive strength. We observed strong correlation (correlation coefficient 0.997) between USV and compressive strength and variance (R2 = 0.87), which meant 87% of variation of compressive strength could be explained by variation of USV for the specimens (which acquired its compressive strength) on 28th day. Compressive strength and USV increased due to hydration reaction leading to C-S-H (Calcium-Silicate-hydrate) gel formation which filled the pores in the concrete matrix. The compressive strength of concrete significantly increased with content of silica nano-particles within the selected range of content (1.5-2.5%), but there is limitation probably due to agglomeration of nanoparticles, which destroyed the salient features nano-particles.