The present work investigates the effect of change in combustion chamber geometry on performance and emission characteristics of single cylinder diesel engine fuelled with mahua biodiesel. Since plant oil derived from the mahua tree has high fatty acids, it undergoes esterification followed by transesterification process to reduce its viscosity. Experiments were conducted using a blend of 20% biodiesel (B20) 40% biodiesel (B40) with diesel and compared with diesel by using two types of combustion chamber geometry, explicitly hemispherical and modified hemispherical combustion chamber. Performance parameters such as Brake Thermal Efficiency (BTE), Brake Specific Fuel Consumption (BSFC) and emission parameters like Unburned Hydro Carbon (UBHC), Oxides of Nitrogen (NOx) were studied from the diesel engine with above mentioned configurations. It is obvious that there is considerable improvement in the performance parameter viz, BTE, BSFC and reduction in UBHC emissions by using the modified geometry piston. However, the NOx emission was found to be higher than that of standard configuration. The results obtained from the blend B20 at modified combustion chamber geometry are on par with diesel and hence mahua biodiesel can be suggested as an alternative fuel for Compression Ignition (C.I) engine with modified combustion chamber geometry.