Biodiesel is an environmentally friendly alternative diesel fuel consisting of the alkyl monoesters of fatty acids. It is obtained from triglycerides through the transesterfication process. The objective of this research was to determine the reason for the higher levels of NOx emissions that have been observed from biodiesel fueled engines. A concept map was developed to show the interrelationships between the fuel and engine variables that affect NOx production. It was determined that a change in combustion timing caused by changes in fuel properties between diesel fuel and biodiesel might be the source of the NOx increase. Tests were conducted to determine the effect of blending biodiesel with diesel fuel on these properties, and to determine the effect of biodiesel fuel properties such as the lower heating value, density, speed of sound, bulk modulus, cetane number, and volatility on the NOx emissions from a diesel engine fueled with biodiesel.;It was found that biodiesel is more dense and less compressible than No. 1 and No. 2 diesel fuels and that the chain length and saturation increase the density, speed of sound, and isentropic bulk modulus. It was also found that the higher oxides of nitrogen emission could be explained by a start of combustion advance. Half of the start of combustion advance originated from a start of injection advance of which about half is due to the automatic timing advance of the pump as it injects more fuel to compensate for the lower heating value of biodiesel and half is due to the bulk modulus, viscosity, and density of the fuel, which show significant differences from pump to pump. The other half of the start of combustion timing advance was due to the higher cetane number of the biodiesel. The higher cetane number in the soybean biodiesel advances the start of combustion and therefore contributes to the increase of the NOx. However, the cetane number of the yellow grease biodiesel was so high that it suppressed the premixed portion of the combustion and lowered the brake specific oxides of nitrogen emission relative to soybean oil biodiesel.