The depletion of fossil fuels and environmental degradation with its emissions motivated the researchers to search for alternatives. Vegetable oils are considered as one of the productive alternative for internal combustion engines because of good combustion properties. Currently, very few commercial devices that utilize bio-diesel combustion for the production of heat, due to the economic viability and limited availability. To attain effective combustion, an effort is made in investigating the effect of change of piston geometry on the Performance & Emission characteristics of bio-diesel of mahua oil. Experiments are conducted to study the effect of varied piston bowl geometry on the performance and emission characteristics of mahua oil biodiesel on four stroke single cylinder diesel engine at constant speed of 1500 rpm for different loads. It is noticed that, at 20% blend of biodiesel of Mahua oil, the performance and emission parameters were improved compared to alone diesel operation. Hence the same optimal blend is adapted for conducting experiments by changing piston bowl geometry of toroidal shape. The experiments are conducted duly ensuring the same compression ratio as that of with standard hemispherical geometry at optimal blend of bio-diesel. It was noticed there is slight improvement in performance parameters and significant improvement in the emission parameters. At optimal blend emissions reduced by 16.4%, 11.4%, and 5.3% of unburnt hydrocarbons (UHC), Carbon monoxide (CO), Carbon dioxide respectively compared to diesel alone operation at rated load. However, there is 8% increase in oxides of Nitrogen (NOx) emission due to attainment of high combustion temperatures with biodiesel. With toroidal geometry, at optimal blend of bio-diesel the emission parameters are further reduced notably but increase in NOx emission is observed compared to standard piston due to high cylinder temperatures and pressures.