The effect of intake oxygen concentration on both the flame temperature and emission of NOx and particulates from a direct injection diesel engine is reported. Nitrogen oxide emissions from a direct injection diesel engine was correlated with calculated stoichiometric flame temperature, yielding -143,000 cal/mole for the overall activation energy for the processes involved. It was found that when In (EIP) was plotted against the inverse of flame temperature at a range in load between 0% to 100% the values of In(EIP) are linear. Analyzing the slope gave a constant apparent activation energy. But the correlation of In(EIP) with flame temperature at 110% load shows smaller activation energy than that at lower load conditions.