The regenerative braking process of hybrid electric vehicles (HEVs) cruising downhill was analyzed. Based on the charging efficiency, temperature model of battery, and efficiency model of the generator, instantaneous and global optimal control strategies were proposed, with the goal of maximum of instantaneous regenerative braking energy and total regenerative braking energy respectively. Regenerative braking energy efficiency on roads of differing slopes and slope lengths were obtained. The results show that global optimal control strategy is better than instantaneous optimal control strategy, and the steeper the slope or the longer the slope is, the more distinct the increase of regenerative braking energy efficiency with a global optimal control strategy is. The influence of battery temperature on efficiency of regenerative braking energy of HEVs was also discussed.