Abstract— There has been an increasing demand for clean energy in the recent days. Fuels used now days are non-renewable fuels, a major reason for environment pollution. Increasing urbanization, increase the demand and also increase the emission from the conventional fuels which created a demand for efficient and alternative energy source. 1/3rd fuel produced in the world is consumed by the transportation sector and a major contributor to greenhouse gases. Production process of energy should be developed which can cater the demand of environmentally safe, renewable and sustainable. Alternate fuels are being researched because of the increasing demand of the convention no-renewable fuel. There have been many renewable energy, but the immediate replacement is biodiesel. Biodiesel is a replacement of the diesel, which is the major fuel in the transportation sector and consists of alkyl esters. Biodiesel are majorly produced from food crops, a major disadvantage and at the same time it increase the production cost. Recent studies show that biodiesel could be produced from waste cooking oil, this does not pose any threat to the food crops. Use of waste cooking biodiesel in the CI engine reduces the HRR and in-cylinder pressure. Many researches also showed that CO, HC, NOx emission reduces with increase in blend ratio. This paper gives a review of the use and characteristics of waste cooking oil biodiesel used in compression ignition engine