Promising biofuels for diesel engines are vegetable oils and fuels derived from them. The advantages of using biofuels in internal combustion engines were considered. The problems of using vegetable oils in diesel engines have been analyzed. It is noted that the main issues of using vegetable oils are their high viscosity and their worst flammability in the combustion chamber of diesel engines. The flammability indicators of vehicle fuels in the combustion chamber of diesel engines were considered. The most important among them is the cetane number of the fuel. The factors influencing the cetane number of vegetable oils have been determined. The dependences of the cetane number of vegetable oils on the flash point in a closed-cup, the content of oleic acid, and the content of saturated fatty acids have been obtained and analyzed. These dependences have been compared with a multi-factorial dependence of the cetane number on the basic triglyceride contents.