The effects of vacuum freeze drying (VFD) pretreatment on poplar (PP), switchgrass (SG), and wheat straw (WS) biochar properties were studied in a muffle at air atmosphere with varied final pyrolysis temperatures (300, 500, and 700 degrees C). Conventional drying (CD) was used as control. As final pyrolysis temperatures increased, biochar yields from both CD- and VFD-pretreated PP, SG, and WS decreased, and yields from VFD-pretreated biomass were lower than those from CD-pretreated biomass. As temperatures increased, the pH of CD-pretreated biochar increased, while the pH of VFD-pretreated biochar did not significantly change. Higher pyrolysis temperatures increased carbon contents in all biochar. According to Fourier-transform infrared spectroscopy (FTIR), CD and VFD pretreatment had a significant effect on biochar's functional groups. Compared to CD pretreatment, VFD-pretreated materials and biochar have an intact porous surface structure and VFD-pretreated biochar contain more micropores, suggesting that VFD pretreatment could generate high-quality porous biochar. (C) 2020 Elsevier Ltd. All rights reserved.