Nano-crystalline Co3O4 and CoO powders have been prepared by a spray pyrolysis approach. The effects of the reaction temperature and initial salts on the crystallinity and phase composition have been studied. Based on the TEM and XRD results, the crystal sizes were in the range of 1-10 nm. SEM and TEM observations also reveal that the nano-powders easily create micron-scale spherical agglomerates. The Co3O4 powders obtained by spraying nitrate solution at 500 degrees C show high specific surface area, which according to the BET method is 82.37 m2/g. The time/temperature phase diagram of cobalt oxides developed from XRD and DTA/TGA analyses shows the existence of a CoO phase at low and high temperature ranges when some specific preparation conditions are applied.