To meet stringent emission norms with internal engine measures, design of piston cavity geometry perform a defining role in air motion, fuel air mixing, combustion and emission formation. A study is performed with the objective to have a better tradeoff between NOx, PM and fuel consumption for a Medium duty, constant speed diesel engine operated with Mechanical fuel injection system. Through simulations in 3D CFD tool the effect of piston cavity geometry on performance and emission of diesel engine is investigated and then validated with actual experimentation.In this exercise efforts are made to reduce emissions in a direct injection diesel engine by changing the piston cavity geometry. The piston cavity geometry and dimensions like torus radius, pip region, cavity lip area, and impingement area have an effect on emission formation. The target was to deliberately split the fuel spray and have a better utilization of available air. It is also observed that larger toroidal radius helps to increase engine performance through enhanced mixing. Above features contribute to reduce both soot and NO without much penalty in fuel efficiency. Simulation results show that optimum shape of the combustion chamber helps to simultaneously reduce emissions.The base cavity simulation is first validated with actual experimental test data of existing 8.8 Liter engine meeting equivalent to TIER III emission norms. The same model is then used to run the simulation with different cavity shapes. After doing simulation for multiple cavity shapes, optimum cavity is selected and actual experimentation is performed. During experimentation In-cylinder combustion parameters, Fuel consumption and Emissions are measured. The experimental data is used to validate the findings from Simulation.