The low heat rejection application in diesel engines has possessions on the fuel consumption, power, combustion efficiency, pollution contents and the fatigue lifetime of engine components. Based on the tests, it was firm that specific fuel consumption was enlarged with biodiesel practice. Exhaust gas temperature and smoke concentration were lessened with biodiesel practice, but enlarged by low heat rejection use. Oxides of nitrogen emission were augmented at a lesser speed, but reduced at higher speed with biodiesel practice and it was augmented by low heat rejection use. In this article, all effects, advantages and disadvantages of low heat rejection application are investigated along with their performance and emissions with vegetable oil based biodiesel.