Abstract: In order to have the biogas formation features of the low and medium rank coal, the biogas formation simulation experiments were conducted with Longk ou lignite and Huainan fat coal as the gas maternal sources. Under the same conditions to control other inftluence factors, while seven group experiments were conducte d, the lignite and fat coal samples as the bottom materials were added to the six group experiments individually. With the statistics and analysis on the gas production vo lume and the gas production rate of the biogas, the paper discussed the biogas formation features of the low and medium rank coal. The results showed that in the 90 d ays biogas formation simulation experiments of the coal, the bio-methane gas formation volume and the gas formation rate of the lignite and fat coal experiment groups would have higher gas formation accumulated volume and gas formation rate as the lower of the coal rank generally. The biogas formation of the experiment groups wo uld have three stages generally. The first stage was the rapid uplifting period, the second stage was the stable dropping period and the third stage was tended to the sta ble period. At the first stage, there were differences between the different rank coal samples and the gas generation of Huainan fat coal was early than Longkou lignite. At the early period of the biogas formatted, the methane strains from bios would be the nutrient solution in the culture medium as the main material and the gas formatio n rate would be high. At the late period of the biogas formation, the methane strains from the bios would consume the organic gas in the coal and thus would cause the gas formation rate decreased.