In this paper, supercritical water gasification was used as a thermochemical conversion technology to produce gaseous products from marine microalgae. The Response Surface Methodology based on Box-Behnken design was selected for modeling and optimizing the effects of variables comprising temperature, microalgae loading and reaction time on gaseous product's composition especially hydrogen generation. The most important variable affecting H-2 production was temperature followed by reaction time and microalgal biomass loading. So, the highest amount of 21.1 mol% H-2 was obtained during SCWG of 1.4 wt% microalgal biomass at 405 degrees C for 45min. At near critical water condition, the effect of two metal-oxide-supported catalysts (NiO/SiO2 and MnO2/SiO2) with different catalyst loadings (50, 75, 100 and 200 wt%) on gas production revealed that, 100 wt% loading of MnO2/SiO2 had the maximum catalytic activity. Gasification at optimum condition with 100 wt% MnO2/SiO2 resulted in maximum hydrogen selectivity and gasification efficiency of 41.5% and 28.6%, respectively. (C) 2018 Elsevier Ltd. All rights reserved.