Cobalt oxide layers was formed on a stainless steel type 304 surface after annealing heat treatment (400 oC / 10 hr) of the previously electrodeposited pure cobalt coating. The influence of heat treatment on surface morphology of pure cobalt films deposited has been studied. Crystallite structure transformation from hexagonal system to cubic system occurs. The coating layer morphology change from nano platelets to nano flecks was observed. SEM and EDAX are used for monitoring the gradually changes in the coated surface and nanostructure formation behavior. The degree of crystallitey and crystallite size changes, due to annealing at 400 oC/10 hrs, was measured by using XRD device. The optical properties were investigated by using UV-IR Spectrometry to study the effect of the formed surface coating at different electrodeposition conditions on the solar energy absorption behavior. The optimal solar absorptance was 98% at visible light wavelength range (400- 750 nm) for the films after annealing heat treatment.