This experimental study investigates the prospective of biogas as fuel in unmodified compression-ignition (CI) diesel engines under biogas-diesel dual-fuel (DF) combustion mode. The premixed air-biogas mixture has been ignited by using diesel as pilot-fuel. The DF mode engine parameters have been investigated with varying the engine load by adjusting the biogas and diesel flow with the individual control mechanism. For the entire tested loads, about 65–69% of diesel mass consumption of the engine has been substituted by biogas under the DF mode. The DF mode consumes lesser friction power than that of the diesel mode. NOx levels have been decreased for the entire loads with biogas fuelled DF mode. The DF mode produced higher CO, HC and CO2 levels to the diesel mode. But, in actual, the NO x+ HC emissions are found within the limit of emission norms, and the amount of CO and CO2 emissions is found much lower when compared to the large volume of CO2 existence in biogas composition. The maximum thermal efficiency of the engine has been observed about 18.66% lower with the biogas-diesel method to diesel way which can be upgraded through advancing the fuel injection timing. This study confirms that it is feasible to use biogas safely being a renewable gaseous fuel in a CI diesel engine with DF mode without operating parameter alterations beyond medium loads. The engine can run for entire loads under DF mode with the achievement of equivalent diesel efficiency, the operating parameters should be adjusted as per the suggestions of this study. © 2021, The Institution of Engineers (India).