In this study, cobalt oxide (Co3O4)/graphene composites were synthesized through a simple chemical method at various calcination temperatures. We controlled the crystallinity, particle size and morphology of cobalt oxide on graphene materials by changing the annealing temperatures (200, 300, 400℃). The nanostructured Co3O4/graphene hybrid materials were studied to measure the electrochemical performance through cyclic voltammetry. The Co3O4/graphene sample obtained at 200℃ showed the highest capacitance of 396 Fg-1 at 5 mVs-1. The morphological structures of composites were also examined by scanning electron microscopy and transmission electron microscopy (TEM). Annealing Co3O4/graphene samples in air at different temperatures significantly changed the morphology of the composites. The flower-like cobalt oxides with higher crystallinity and larger particle size were generated on graphene according to the increase of calcination temperature. A TEM analysis of the composites at 200℃ revealed that nanoscale Co3O4 (~7 nm) particles were deposited on the surface of the graphene. The improved electrochemical performance was attributed to a combination effect of graphene and pseudocapacitive effect of Co3O4.