The effects of hydrogen (H₂) ratio on combustion and emission characteristics in a H₂/diesel dual-fuel engine were investigated. Dual-fuel strategy was applied to improve the control of combustion phasing. The combustion phasing was retarded with increasing H₂ fraction. This can be explained by both reduced diesel concentration and chemical effect of H₂, which reduce the heat release rate during the low temperature reaction stage. Hydrocarbon and carbon monoxide emissions of the engine were decreased drastically when H₂ ratio was increased.