There is substantial interest in the application of biomass as a renewable fuel or for production of chemicals. Flax straw can be converted into valuable chemicals and biofuels via liquefaction in sub-critical water. In this study, the yield of furfural and the kinetics of flax straw liquefaction under sub-critical water conditions were investigated using a high-pressure autoclave reactor. The liquefaction was conducted in the temperature range of 175-325 degrees C, pressure of 0.1 MPa-8 MPa, retention time in the range of 0 min-120 min, and flax straw mass fraction (w(F)) of 5-20 %. Also, the effect of acid catalysts on furfural yield was studied. The kinetic parameters of flax straw liquefaction were determined using nonlinear regression of the experimental data, assuming second-order kinetics. The apparent activation energy was found to be 27.97 kJ mol(-1) while the reaction order was 2.0. The optimum condition for furfural yield was at 250 degrees C, 6.0 MPa, w(F) of 5% and 0 retention time after reaching set conditions. An acid catalyst was found to selectively favour furfural yield with 40% flax straw conversion. (C) 2014 Elsevier Ltd. All rights reserved.