Detection of bio chemical species in a liquid is important for research and technical purposes. In the last decade, metallic nanoparticles have been shown to be potential for such measurement. Resonance wavelength (or frequency) of free electron on metallic nanoparticle surface is very sensitive to minute changes on its surrounding environment. Change in surrounding medium shifts the resonance wavelength that depends mainly on the size and geometry of metal nanoparticle. We numerically optimized size and aspect ratio of silver and gold nanoparticles to obtain the highest sensing sensitivity. Here, sensitivity is defined as the slope of resonance wavelength on refractive index of liquid. We found that size, aspect ratio, and material strongly affect sensing sensitivity of metal nanoparticle based sensor.