A method for thermal management of Li-ion battery pack with the application of various flexible form-stable composite phase change materials (CPCMs) is proposed, and investigated both numerically and experimentally. Flexible CPCMs embedded in battery pack lower the thermal contact resistance, thereby improving the thermal control performance. Experimental results show that with the application of flexible CPCMs, the battery temperature drops by 18 degrees C at 10 C discharge rate. In addition, flexible CPCM has better thermal control performance than the case of conventional CPCMs. This allows Li-ion battery pack to safely work for an extended period of time without exceeding upper temperature limits. The thermal control performance is observed to vary considerably with three factors: the phase change temperature of flexible CPCMs, working condition, and the ambient temperature. One flexible CPCM with phase change temperature of 33 degrees C is found to be optimal for small power levels and low ambient temperatures. Another flexible CPCM with 47 degrees C of phase change temperature is suitable for high power levels, short-term high heat fluxes, and high ambient temperatures. The numerical results are in reasonable accordance with experimental ones.