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.