The term biodiesel means the monoalkyl esters of long-chain fatty acids made from biolipids such as vegetable oils, animal fats, or algae. Chemical reactivity of biodiesel can be divided into oxidative and thermal instability. Many of the biolipids contain polyunsaturated fatty acid chains in that their double bonds have high chemical reactivity. The oxidative and thermal degradation occurs on the double bonds of unsaturated aliphatic carbons chains in biolipids. Oxidation of biodiesel results in the formation of hydroperoxides. The formation of the hydroperoxide follows a well-known peroxidation chain mechanism. The olefinic unsaturated fatty acid oxidation is a multi-step reaction process where primary products (conjugated diene hydroperoxides) decompose and chemically interact with each other to form numerous secondary oxidation products. The oxidative and thermal instability are determined by the amount and configuration of the olefinic unsaturation on the fatty acid chains. The viscosity of biodiesel increases with the increase of thermal degradation degree due to the trans-isomer formation on double bonds. The decomposition of biodiesel and its corresponding fatty acids linearly increases from 293 K to 625 K. The densities of biodiesel fuels decreased linearly with temperatures from 293 K to 575 K. The combustion heat of biodiesel partially decreases with the increase of thermal degradation degree.