Nano-and micro-sized ZnO samples were synthesized by the sol–gel method. Physical and thermal stability of ZnO samples were tested in supercritical water (SCW) and supercritical water gasification (SCWG) of formaldehyde. ZnO calcined at 350 °C (ZnO-350) has spherical particles with an average diameter of 150 nm. After SCW and SCWG treatments, the particle size of ZnO-350 increased and its roughness decreased as a result of agglomeration. Increasing the calcination temperature of ZnO from 350 to 900 °C increased crystallinity and the estimated crystal size of ZnO. The particles of ZnO calcined at 900 °C converted to hexagonal particles from spherical particles by the dissolution of coke precursors in SCW. Compared with the non-catalytic gasification of formaldehyde, the presence of ZnO increased the conversion of formaldehyde and hydrogen formation. The increasing calcination temperatures of ZnO enhanced hydrogen formation, which might be due to the formation of oxygen vacancies on the ZnO1-x.