A diesel combustion calculation code has been developed and it is currently used for our actual engine development. But the code has one weak point. In some injection patterns including pilot injection or pre injection with a small amount of fuel, it can’t predict ignition delay time precisely. That is because the ignition delay time is figured out by the Livengood-Wu integral method. Therefore the changing from the simple method to chemical kinetics calculation using an elementary chemical reaction model was tried out. The overview of our chemical calculation and some knowledge from observing the ignition delay time differences between the current simple method and the chemical kinetics calculation are shown in this report.