Homogeneous spherical W-Co salt powders were made by spray drying of aqueous solution of ammonium metatungstate(NH 4 ) 6 (H 2 W 12 O 40 )?4H 2 O,AMT) and cobalt nitrate hexahydrate(Co(NO 3 ) 2 ?6H 2 O) . The thermal decomposition process of spray dried W-Co salt powders was studied by TG, XRD, SEM, TEM and FT-IR. Spray dried W-Co salt powders were calcined for 1 hour in the temperature from 350 ? C to 800 ? C in atmosphere of air. At the temperatures over 600 ? C , spherical CoWO 4 /WO 3 composite oxide powders were obtained. The primary particle size of W/Co composite oxide powders increased with increasing thermal decomposition temperature due to the particle growth. The observed crystallite size by TEM was in the range of 60nm and that of CoWO 4 calculated by Scherrer's formula at 800 ? C was smaller than 55nm. The crystallite site was identified by XRD and TEM.