In this research, semi-coke samples were produced in a modified microwave oven at different final temperatures at a constant incident microwave power (550 W). The pore structure of raw oil shale (OS) and semi-coke samples was measured by the method of low-temperature adsorption of nitrogen. The obtained adsorption/desorption isotherms of these samples were similar and could be categorized as type II isotherms in the IUPAC classification, and all hysteresis loops produced by OS and all semi-coke samples are very similar to H3 hysteresis loops. Specific surface area was calculated based on BET equation, and pore size distribution was calculated by BJH method. The analysis results showed that the final temperature has a great influence on specific surface area, specific pore volume and the development of mesopores. As compared with conventional pyrolysis, pore size distribution curves have two visible peaks near 2 nm and 4 nm.