This research aimed to investigate the potential assessment of normally used cooking oil (Mustard oil (MO) and Rice bran oil (RBO) as a biodiesel after transesterification process and find the optimum conditions for this process. Unused oil (0 th sample/control) and used oil (after10 th cycle of heating and cooling) were used as experimental sample in this study. Two temperature ranges were used i.e. 27±2°C and 150±5°C for cooling and heating of oil samples respectively. Fuel quality indexes (FQI) were analyzed in reference of biodiesel for used and unused cooking oil samples (control). Similarly, control and samples after 10 th cycle with selected cooking oils samples (1 st and 10 th only) after transesterification process were also evaluated for fuel quality. On comparison, before and after transestrification with 10 th cycle of heating and cooling oil samples of MO and RBO, shows a remarkable change in the value of density, viscosity and little change with iodine number and flash point in comparison to control and 1 st cycle of heating and cooling of oil samples. Result showed a similar trend in value like commercial biodiesel after evaluating FQI. On comparative basis, RBO (10 th cycle sample after transestrification) was found to be more feasible as diesel fuel. © 2018 Institution of Engineering and Technology. All rights reserved.