Batch-wise supercritical water gasification (SCWG) of two types of microalgae was carried out in the absence and the presence of nickel catalysts in a laboratory scale reactor at 385 °C and 26 MPa. The microalgae used include Chlorella vulgaris (high protein and low carbohydrate contents) and Scenedesmus quadricauda (low protein and high carbohydrate contents). Time course trend of the total carbon-based gas yield of C. vulgaris and S. quadricauda was similar, showing asymptotic convex changes that may be attributed to the positive reaction order of SCWG of microalgae. In the presence of catalyst, the yield increased to 80-90 with time in an asymptotic way, whereas it reached only 12 in the absence of catalyst. For non-catalytic SCWG, the predominant gas product was carbon dioxide, which may be formed by decarboxylation of biomass. For catalytic SCWG, the major gas products were CH4, CO2, H2, and CO, of which composition can be explained by successive reactions of (1) hydrolytic breakdown of macromolecules to smaller organics, (2) gasification of organics to CO, CO2 and H2, (3) shift reaction of CO, and (4) methanation of CO and CO2, and their equilibrium. © 2015 Elsevier B.V. All rights reserved.