The aim of this research is to study the effects of neat biodiesel (B100) and pure jatropha oil on engine performance, black smoke density, fuel consumption and durability of engines. Two 14-horse power, single cylinder diesel engines were dedicated for the experiment using those fuels. From the performance test, when comparing BDF and jatropha oil with diesel, the engine performances were slightly different with a small increase of fuel consumption. It is noticeable that black smoke measured from the engines using both biodiesel and jatropha oil can be hugely reduced. However, in the case of field test when each engine was connected to power water pumps in order to determine the long term effects, the engine that was fueled with jatropha oil presented some problems. The injector and fuel filter were clogged enough to cause engine malfunction. The parts were then replaced and petroleum diesel fuel was added at an increment of 20% in the blended fuel to reduce the concentration of jatropha oil until the engine could run continuously again. It was found that the highest amount of jatropha oil could be used was a blend between jatropha oil and diesel fuel of 60:40 by volume. In the case of the engine using biodiesel, the field test could be performed without any engine problem for 500 hours. The engine performance and black smoke after 300- and 500-hour field test were also measured while the fuel line system was investigated. All results are analyzed and discussed in this paper.