Abstract: Biomass pyrolysis has attracted significant interest in recent years. However, traditional pyrolysis technologies such as fluidized bed type of pyrolysis technique require biomass to be ground into fine particles which adds a lot of energy input and cost. In addition, bio-oil and syngas products produced are often contaminated by the incomplete reacted biomass materials. Microwave pyrolysis helps solve those problems, however, just like other pyrolysis technologies, the bio-oil products from current pyrolysis technologies including microwave are still very complex in composition, acidic, and not stable, which prevent the commercialization of the pyrolysis technology, even though the cost of bio-oil production from pyrolysis is much less than that of biological approach. The study intends to help solve this exact problem. Corn stover was used as feedstock, and the effects of catalysts on the product selectivity of microwave assisted pyrolysis were investigated. Many kinds of acids, alkali, metal oxides, and salts were tested as catalysts for pyrolysis. The bio-oil compositions were analyzed with GC-MS. It was found that when corn stover pellets were pre-mixed with some kinds of acids, and/or chloride salts prior to microwave assisted pyrolysis, the components of the liquid bio-oil products were very simple. At 8 g catalysts per 100 g corn stover, the GC-MS total ion chromatograms of the bio-oils show only one major furfural peak accounting for about 80% of the area under the spectrum.