An aqueous sol dispersing nano crystallites of WO 3 .2H 2 O was subjected to a centrifugal treatment for 1 or 10h, to obtain two kinds of WO 3 .2H 2 O gels with very different morphology of crystals. The WO 3 thick film devices fabricated with these gels were investigated for several properties including NO 2 sensing characteristics. It was found that the devices prepared under proper conditions exhibited excellent NO 2 sensing properties to cover environmental standard of NO 2 . The sensitivity to NO 2 as well as the rate of response depended markedly on calcination temperature of the devices. The morphology of WO 3 crystals, i.e. lamellae or disks, was almost the same as that of WO 3 .2H 2 O gel unless calcination temperature exceeded 500 o C. Based on the surface analysis using N 2 adsorption technique, the above changes were estimated to result from a change in microstructure of the WO 3 crystals.