The phase-varying mechanism of the ion current observed in a Homogeneous Charge Compression Ignition (HCCI) engine is investigated to achieve ion current-based combustion phasing. By integrating the gasoline flame ionization mechanism with the HCCI combustion model, the mechanisms affecting the ion formation and recombination processes are analyzed, and the relationship between the phases of ion current and combustion event is studied. Modeling results indicate that the formation rate of H3O+ ions is mainly affected by the combustion boundary conditions. However, the ion recombination rate of H3O+ ions is mainly dependent on the concentration of these ions. In the presence of the above mechanisms, the phase-varying tendency of the ion current is found to be similar to the variations in the combustion phase, but the offset between these phases will vary when the combustion boundary condition changes. As the equivalence ratio becomes low, the rate of H3O+ formation is decreased and the ion recombination rate decreases even more, due to the reduced ion concentration. Therefore, the inflection point of the ion current curve, dI(max), is retarded even further compared to the combustion phase CA50. In addition, a larger phase offset between dI(max) and CA50 is observed when the intake temperature is lower. All of the above modeling predictions agree well with the experimental results. (C) 2013 Elsevier Ltd. All rights reserved.