The paper deals with an analysis of optimal battery state-of-charge (SoC) trajectory profile for plug-in hybrid electric vehicles (PHEV). The case of blended operating regime is considered, where hybrid driving mode is enabled during the whole trip. The analysis relates to a SoC trajectory length minimization pattern observed in optimal SoC trajectories obtained by using dynamic programming-based optimisation of PHEV powertrain control variables. The emphasis is on analytical interpretation of the optimal SoC trajectory pattern from the perspectives of simplified battery-only system and the whole PHEV powertrain. Also, the impact of engine specific fuel consumption characteristic on the optimal SoC trajectory solution is analysed.