This paper investigated the effect of hydrogen (H2) addition on the combustion process of a heavy-duty diesel engine. The addition of a small amount of H2 was shown to have a mild effect on the cylinder pressure and combustion process. When operated at high load, the addition of a relatively large amount of H2 substantially increased the peak cylinder pressure and the peak heat release rate. Compared to the two-stage combustion process of diesel engines, a featured three-stage combustion process of the H2–diesel dual fuel engine was observed. The extremely high peak heat release rate represented a combination of diesel diffusion combustion and the premixed combustion of H2 consumed by multiple turbulent flames, which substantially enhanced the combustion process of H2–diesel dual fuel engine. However, the addition of a relatively large amount of H2 at low load did not change the two-stage heat release process pattern. The premixed combustion was dramatically inhibited while the diffusion combustion was slightly enhanced and elongated. The substantially reduced peak cylinder pressure at low load was due to the deteriorated premixed combustion.