AbstractBiodiesel fuel and biogas fuel as promising alternative energy sources for dual‐fuel‐mode diesel engine attract more researchers. Therefore, this study highlights the dual‐fuel mode (DFM) operation of diesel engine using biodiesel fuel from used cooking oil and simulated biogas fuel with different methane contents (M40, M60, M80, and M100) on the combustion, rate of heat release, combustion stability, and performance. The observation of diesel engine was conducted by varying the torques from 3.5 Nm, 10.5 Nm, 17.6 Nm, and 24.6 Nm and by varying the engine speeds from 1800 rpm, 2000 rpm, 2200 rpm, 2400 rpm, and 2600 rpm, respectively. It was found that in DFM at a low torque the thermal efficiency decreased, but biogas replacement, biogas energy, and sfc values increased. The relation of the results of biodiesel replacement, biogas energy ratio, and brake thermal efficiency with methane content ratio in DFM show that the methane content ratio has the maximum effect in DFM. In addition, the carbon dioxide content in the biogas can enrich the brake thermal efficiency. The combustion stability of all conditions (1.7%‐4.89%) is still acceptable.