The process of thermal debinding of injection molded water atomized 316L stainless steel powder compacts was studied in a vacuum and hydrogen atmosphere using a mercury porosimetry and the thermogravimetric analysis method. The vacuum debinding rate was investigated under the conditions of different binder compositions and temperature-increase rates, especially at the low temperature stage. The process of thermal debinding in the hydrogen atmosphere was compared with that in vacuum. It was found that the binder removal rate was much higher in vacuum than that in the hydrogen atmosphere under the same experimental conditions. The mercury porosimetry was used to measure the pore structure evolution. The results of the mercury porosimetry indicated that the pore structure evolution in injection molded compacts during thermal debinding in vacuum was greatly different from that in hydrogen atmosphere. A fine inter-connected pore structure developed as a result of the evaporation and removal of the low molecular binder components at the early stage of debinding. The reason for the difference in binder removal rate and the inter-connected pore structure in these two debinding environments lies in the difference of the vapor pressure drop, which was the vapor pressure difference between that in the molded compacts and that in the ambient environment. At the different thermal debinding stages and environments, the pore structure evolution and changes in the mean pore size in the thermally debound compacts were also different. From the low debinding temperature stage to the intermediate temperature stage of about 300 8C, the pores in the thermally debound compacts in the hydrogen atmosphere were finer and the pore size distribution was more homogeneous than that in the vacuum. At the final debinding stage, the pore size distribution was almost the same in the hydrogen atmosphere as in the vacuum.