Two kinds of MoO3/SiO2 catalysts, MoO3-I and MoO3-S, were prepared by impregnation and sol-gel method, respectively. When MoO3 loading was increased, formation of MoO3 crystals was observed to begin at a MoO3 loading of 8 and 16 wt% with MoO3-I and MoO3-S, respectively. The highest yield of formaldehyde from methane oxidation was attained also at those critical values of MoO3 loading of 8 and 16 wt% over MoO3-I and MoO3-S, respectively. It is suggested that the active species for formaldehyde formation is well dispersed molybdenum oxide clusters on SiO2 support : the optimum dispersion of the clusters affords the highest activity for formaldehyde formation.