The role of thermal mass is very important in natural ventilated buildings Thermal mass can be classified as external thermal mass and internal thermal mass In this paper a heat balance model for naturally ventilated building is developed The effect of external thermal mass is introduced by harmonic response method and the effect of internal thermal mass is calculated by virtual sphere method which can lump different shapes and types into one virtual sphere The effective convection heat transfer coefficient is presented to represents the effect of uneven distribution of internal thermal mass temperature Through comparison with L1 s model [1] and numerical calculation it was found that the model is more suitable than the others especially in estimating decrement factor of indoor air temperature The errors of time lag become smaller when the fluctuation of indoor air temperature is smaller Based on the model a simple tool is developed to evaluate the indoor air temperature and required internal thermal mass for certain naturally ventilated buildings (C) 2010 Elsevier B V All rights reserved