Only recently has coalbed gas been recognized as a new nonconventional natural gas resource. Coalbed gas is generated and stored in coal by two distinct processes during the coalification process: biogenic and thermogenic. Biogenic gases can be divided into primary biogenic gas and secondary biogenic gas depending on differences of geologic time and source rocks. Primary biogenic gas is gerenated at lower coal rank and is difficult to perserve. Secondary biogenic gas is usually associated with late-stage groundwater system and bacteria activity. Generation of thermogenic coalbed gas begins at Ro= 0.5%~0.8%. Coalbeds are source rocks for liquid hydrocarbons and gases. Compared with gases generated from dispersed types and kerogen or type kerogen, isotope and composition of gases produced from coalbeds are highly variable. The variation indicates many and complex controls on composition and origin of coalbed gas, including maceral composition, coal ranks, gas generation processes and depths of burial(temperature and pressure). In addition, hydrologic setting and secondary processes also influence composition of coalbed gas.