This study investigated microwave pyrolysis of corn stover with particle sizes from 0.5 to 4 mm and determined the effects of reaction temperature and time of pyrolytic conditions on the yields of bio-oil, syngas, and biochar. Mineral and GC/MS analysis were used to study the pyrolysis of corn stover. A prediction model was satisfactorily developed to describe the biofuel conversion yield as a function of reaction temperature and time. Corn stover with different particle sizes was found to be similarly pyrolyzed by microwave heating. The research results indicate that thermochemical conversion reactions can take place rapidly in large-sized corn stover by using microwave pyrolysis. Mineral analysis indicates that most minerals stayed with the biochar. GC/MS analysis indicates that the bio-oil contained a series of important and useful chemical compounds: phenols, aliphatic hydrocarbons, aromatic hydrocarbons, and furan derivatives. These chemical compounds evolved were related to the pyrolysis conditions.