The combustion process in high-speed direct-injection diesel engines is characterized by random turbulent mixing between turbulent eddies having different fuel concentrations. Nitric oxide and soot are formed in hot eddies and fuel-rich eddies. In the present study, the authors elucidate the diesel combustion process, from the viewpoint of such heterogeneity and turbulent mixing, by analysis of high-speed flame photographs. Based on this study the following points are suggested: jet-like flames are formed just after ignition but soon disintegrate into random turbulent flamelets as each flame quickly expands. In the middle and later stages of combustion, uniform and isotropic turbulent motions prevail over the entire space, gradually decaying with time. Such turbulent motions favor the destruction of fuel concentration heterogeneity. Gas expansion due to combustion enhances such random motions, and the swirl prevents their early decay in every burning stage. These observations are basically consistent with those of the stochastic model previously proposed by the present authors. Based on that model, nitric-oxide and soot formation are discussed from the viewpoint of turbulent mixing.