The formation of biogenic coalbed methane is stoichiometrically limited by relative deficiency of hydrogen in coal. As straw is rich in hydrogen and can be easily degraded by microorganisms, it is suggested that the co-degradation of coal and straw to produce biogenic methane can overcome these limitations. However, the factors affecting methane production in co-degradation of coal and straw were unclear. Here, the type and part of straw and the particle size of coal and straw were evaluated with respect to methane production from Qinshui Basin coal. The results showed that the biomethane yields in co-degradations of coal and different types of straw were all higher than that in degradation of only coal or only straw with the yield from rice and coal 25.36 times greater, the yield from sweet sorghum and coal 22.70 times greater, the yield from wheat and coal 19.90 times greater, and the yield from corn and coal 1.74 times greater. The co-degradation of coal and stems also showed a better enhancing effect than that of coal and roots and coal and leaves based on biogenic methane production. With respect to particle size, the coal had a greater influence on biomethane production than the straw. The highest methane production was obtained in the co-degradation with 0.125-0.150mm of coal and 5mm of rice straw. The affects were also shown to not correlate well with the additional of hydrogen. Therefore, it is more likely that the enhanced methane production is a result of stimulating the microbial biodegradation of coal. Further efforts would be made to find the specific compounds in straw which interact with coal directly, in order to better understand the mechanism of co-degradation between coal and straw, and make it simpler to improve biomethane production.