Abstract: In order to find out the differences of compost maturities among different sized MSW and the size that has the most significant effect in composting, the mixed municipal solid waste (MSW) of 15～80 mm used in Nangong compost plant was divided into five groups according to different sizes: 15～30, 30～40, 40～50, 50～60 and 60～80 mm, and their physical compositions were classified. The multiple regression models have been founded by using the statistics analysis of the compost maturity index while assaying NH+4-N, NO-3-N, WSC, germination index(GI), pH, EC and C/N ratio at 0, 8, 18 and 42 days’ composting process, respectively. The results indicated that: large grains of MSW which consisted of a great number of nondegradable components were more suitable for landfill and incineration treatments; those maturity indexes of MSW in different sizes were different and the effect from the group of 15～30 mm was the most remarkable in the whole composting process, which was indicated by the indexes of GI and C/N ratio and the frequency and the coefficient in the models. This research has provided the necessary backup data and scientific foundation for material pretreatment and technology optimization in composting process in the future.