The substitute to diesel fuels desires to be theoretically and environmentally adequate, and economically viable. Vegetable oil is one of several alternative fuels designed to extend the efficacy of petroleum, the flexibility and cleanliness of diesel engines. In this paper comparative experiments were carried out to measure the carbon monoxide, hydrocarbons, carbon dioxide and oxides of nitrogen emission level on Diesel engine with SCR technique using diesel fuel and Biodiesel blends of Jatropha, Pongamia and Neem (J20D80, P20D80 and N20D80) and the  emission characteristics were analyzed. The results from the experiments prove that vegetable oil and its blends are potentially good substitute fuels for diesel engine in the near future when petroleum deposits become scarcer. The smart technologies deliver benefits to multiple interests, including an improved economy, and a positive impact on the environment and governmental policies. Continuous availability of the vegetable oils needs to be certain before embarking on the major use of it in I.C. engines. Domestically produced vegetable oil will help to reduce costly petroleum imports and the development of the vegetable oil based bio-diesel industry would strengthen the rural agricultural economy of agricultural based countries like India.