Due to good energy storage capacity with the large latent heat, phase change material (PCM) can be applied as interior thermal mass to improve indoor thermal comfort degree in a passive solar room. The traditional approach on the application of PCMs as interior thermal mass is case-study, by which the optimal characteristic of PCM can only be obtained based on lots of cases studied. A new and simple analytical method to get the optimal phase change temperature and latent heat of the interior PCM for a passive solar room is presented in the paper. The estimated optimization results are compared and agree well with that obtained by optimizing non-linear thermal mass in an inverse problem. It is also proved by simulation that the passive solar room with the estimated optimal PCM can reach indoor thermal comfort. (C) 2011 Elsevier Ltd. All rights reserved.