Phase change material (PCM) shows excellent potential to prevent heat gain and reduce the cooling load of buildings. However, the selection of suitable PCM for a given climatic condition becomes important for the effective thermal performance of the building envelope. In the present work, based on the key thermal responses i.e. total heat gain, time lag, and decrement factor, a new Measure of Key Response index (MKR index) is introduced that is used as an evaluation criterion for selecting a suitable PCM for building envelope. For the same, a numerical analysis is carried out for seven cities representing different climatic zones of India during the six months of the summer season from March to August. Seven commercially available PCMs are considered for the analysis. The most suitable PCM for each city is determined based on the MKR index. The results indicated that the selection of suitable PCM can lead to a more than 50% reduction of total heat gain for all seven cities.