This disclosure provides a method of controlling a diesel engine, which includes executing a premix combustion pattern fuel injection containing one or more injections at a first timing, while introducing an amount of EGR gas according to an operating condition of the engine into a cylinder, when an engine load is low, to operate in a premix combustion mode, executing a diffusion combustion pattern fuel injection containing one or more injections at a second timing that is later than the first timing, when the engine load is high, to operate in a diffusion combustion mode, and executing a diffusion combustion pattern fuel injection at a third timing that is later than the second timing, while introducing the amount of EGR gas according to the operating condition of the engine into the cylinder, when shifting from the diffusion combustion mode to the premix combustion mode with a decrease in engine load.