Microwave liquefaction of precipitated Kraft lignin was carried out in polyethylene glycol (PEG) and glycerol (G) mixed with or without H2SO4 as catalyst. The influences of some independent variables on the yield and hydroxyl index were discussed. The viscosity, molecular distribution (GPC), and the types of volatiles measured by gas chromatography-Mass spectrometry (GC-MS) of all the liquefied products were determined. Response surface methodology (RSM) was used to optimize liquefaction conditions. Based on the results, lignin/solvents (wt%), catalyst/solvents (wt%), and reaction time were chosen as independent variables for a central composite design (CCD). The optimal liquefaction conditions were as: 20wt% of lignin, 3wt% of catalyst at 5min with yield and hydroxyl number of 95.27% and 537.95mg KOH.g(-1), respectively. Functional groups (measured by ATR-IR [attenuated total reflectance - infrared]) and the thermal degradation (TGA) of optimized bio-polyol and precipitated kraft lignin were determined.