Sub/supercritical water liquefication (SCWL) is a water-based thermochemical technology as well as an environmentally friendly treatment by converting wet feedstock into bio-energy. In the present study, a systematic investigation of SCWL of lignite was carried out covering a temperature range between 320 and 440 degrees C when residence time increased from 5 min to 40 min. The highest bio-oil oil yield of 34.3% with solid residue of 52.7% was obtained at 440 degrees C for 5 min. Phenol derivatives, carboxylic acids, long chain hydrocarbons, ketones, and naphthalene were the main bio-oil composition through FTIR and GC-MS analysis. Gas yields and their exact compositions were also determined and CO2 was the dominate gas product but the percentage of CH4 became significant at severe SCWL conditions. A conclusion was drawn that fast liquefaction (e.g. 5 min) at relative higher temperature (e.g. 400 degrees C) which avoid excessive gasification and repolymerization reactions was an optimization strategy for high yield bio-oil production from SCWL of lignite. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.