An experimental study of simultaneous sintering of several particles has been carried out using spherical polymer grains. Considering rotational molding condition, coalescence of several grains in contact, happens simultaneously on internal surface of the mould. Theoretical model based on the effect of surface tension and viscosity can accurately predict the coalescence of a pairs of grains. However, it was observed in this study that coalescence rate changes with presence of neighboring grains and their position and the theoretical model pro-posed for two grains, is not able to predict the coalescence rate of mutli-grains. Based on this finding, we have modified this model with taking into account the effect of neighboring particles in the sintering rate of multi-grains. Obtained modified model is capable of predicting the multi-grains sintering rate observed in this study. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 2784-2792, 2011