Hydrothermal liquefaction is a thermochemical technique for obtaining crude bio-oil from lignocellulosic biomass with moderate temperature and pressure. The crude bio-oil can then be upgraded to various biofuels and bioproducts. Hydrothermal liquefaction is amenable to use of biomass feedstocks that have high-moisture. The overall goal of this research is to demonstrate the effectiveness of white rot fungus (WRF) as a pretreatment option in the production of bio-oil from switchgrass through hydrothermal liquefaction. If WRF is an effective pretreatment, it could be a cost-effective option for commercialization, allowing hydrothermal liquefaction to be used on an industrial scale to produce high quality bio-oil capable of replacing some of the fossil fuel liquids used today. This thesis specifically focuses on the investigation of the effects of particle size and culture time on lignin degradation using Phanerochaete chrysosporium as a pretreatment method on switchgrass. In addition, the conversion efficiency of WRF treated switchgrass was compared to that of torrefied switchgrass and untreated switchgrass after the pyrolysis conversion process. The results indicate that WRF outperforms torrefaction as a pretreatment method for the conversion of sugar-based components, thus may be an attractive alternative for fermentation conversion processes, but probably not for thermochemical processes.