Abstract The effect of NiO contents on the microstructure of mesoporous NiO-Gd0.25Ce0.75O2-x (NiO-GDC) composite for intermediate temperature solid oxide fuel cells (IT-SOFC) was investigated. Mesoporous NiO-GDC powders with different NiO contents were synthesized by self-assembly hydrothermal method using tri-block copolymer, Pluronic F127, as a structure directing agent. Grain growth/agglomeration behaviors of NiO particles and changes of mesoporous structure of GDC particles were characterized by microstructural analyses. NiO-GDC powders were composed of GDC nano particles with ordered mesopore inside the particles and octahedral NiO grains with truncated-edges. As the amount of NiO increases, specific area value of mesoporous NiO-GDC was decreased, and the agglomeration/growth behavior of NiO grains was accelerated.