The objective of this study is to analyze the effect of biogas on the operating characteristics and exhaust gas emissions of a CI diesel engine using a mixture of biogas and diesel. A 5.2 kW Kirloskar made single cylinder, water cooled, direct injection diesel engine is altered for biogas mode operation by connecting a venturi gas mixer at air inlet manifold. Azolla and corn husk feedstocks are employed to produce biogas. Biogas when used in engine reduced the ignition delay which resulted in reduced smoke and hydrocarbon emission, also improved combustion phasing, and increased the peak heat release rate. Around 1.58% increase in brake thermal efficiency was found for Azolla biogas mode operation.