A crack-free asymmetric membrane of perovskite-type oxide (La0.6Sr0.4Co0.2Fe0.8O3-delta) was successfully prepared by coating a slurry containing La0.6Sr0.4Co0.2Fe0.8O3-delta powders directly on the surface of a green support of the same composition, followed by sintering. It was found that crack-foe asymmetric membranes could be obtained by controlling the powder concentration of the slurry in the range of 15-25 wt.% After sintering, the crystal phase of the top layer of asymmetric membranes prepared was the same as that of powders, which were of the cubic perovskite phase. The nitrogen permeability and SEM photograph of the support showed that the support was porous, and the gas-tight test and SEM demonstrated that the top layer of asymmetric membrane was dense and crack-free. The asymmetric membrane prepared, whose dense top layer was 200 mum thick, exhibited about three to four times as high an oxygen flux as a 2 mm dense sintered disc.