Microalgae (N. chlorella) hydrothermal liquefaction (HTL) was conducted at 320 degrees C for 30 min to directly obtain original aqueous phase with a solvent-free separation method, and then the supercritical water gasification (SCWG) experiments of the aqueous phase were performed at 450 and 500 degrees C for 10 min with different catalysts (i.e., Pt-Pd/C, Ru/C, Pd/C, Na2CO3 and NaOH). The results show that increasing temperature from 450 to 500 degrees C could improve H-2 yield and TGE (total gasification efficiency), CGE (carbon gasification efficiency), HGE (hydrogen gasification efficiency), TOC (total organic carbon) removal efficiency and tar removal efficiency. The catalytic activity order in improving the H-2 yield was NaOH > Na2CO3 > None > Pd/C > Pt-Pd/C > Ru/C. Ru/C produced the highest CH4 mole fraction, TGE, CGE, TOC removal efficiency and tar removal efficiency, while NaOH led to the highest H-2 mole fraction, H-2 yield and HGE at 500 degrees C. Increasing temperature and adding proper catalyst could remarkably improve the SCWG process above, but some N-containing compounds were difficult to be gasified. This information is valuable for guiding the treatment of the aqueous phase derived from microalgae HTL. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.