In the present research work, effect of compressed natural gas (CNG) flow rate, on the performance of modified dual fuel engine powered with two biodiesels is studied. Biodiesels derived from Argemone oil and their B20 blends are used as pilot injected fuels while CNG is used as the manifold inducted fuel in the modified dual fuel engine. Further influence of CNG gas flow rate on the modified dual fuel engine by using conventional mechanical fuel injection for optimal performance of engine is investigated. Increasing the CNG gas flow rates reduces brake thermal efficiency (BTE), smoke and nitric oxide (NOx) emissions while, CO (Carbon monoxide) and HC (Hydrocarbon) emissions increased. The engine operated with present fuel combinations can facilitate partial as well as complete substitution for fossil fuels and reduce the greenhouse gas emissions.