In this study, biomass conversion of chlorella sp. microalgae was investigated in the catalytic supercritical water gasification (SCWG) process. The catalytic effect of five different graphene-supported metal nanoparticle catalysts (Ni/rGO, Cu/rGO, Co/rGO, Mn/rGO and Cr/rGO) were evaluated on the composition of syngas. The impact of main process factors including catalyst loading (40, 70, and 100wt%), and temperature (355-405 degrees C) was studied on the SCWG process over Ni/rGO catalyst as the most efficient catalyst among the selected metal nanoparticles. The findings showed that at the optimum Ni/rGO loading of 70wt% (at temperature of 380 degrees C), the hydrogen yield increased by a factor of 5.8 in comparison to absence of Ni/rGO catalyst. At the optimum temperature of 405 degrees C (Ni/rGO loading of 40wt%), hydrogen selectivity of 31.9% was obtained. The results revealed that increment of Ni/rGO catalyst loading extremely reduced the solid residue and tar formation, which is desired in the SCWG process. Moreover, Characterizations proved the stability of prepared catalysts at supercritical condition.