The main objective of this work is to understand steam gasification performance of municipal solid wastes (MSWs). The thermodynamic analysis was carried out by using seven different kinds of MSWs as the feedstock with the temperature ranging from 773 to 1773 K and steam to MSW ratio ranging from 0.5 to 1.5. It is summarized that temperature and the effective mole ratio of H/C (R-H/C) are two key factors, and lower temperature and/or larger R-H/C result in a larger mole ratio of H-2/CO. The model predictions agree very well with the referenced experimental results, which can predict steam gasification performance of MSWs.