The effect of the unfairness in ship bottom plate produced by fillet weld of floor plate and bottom shell on the buckling strength of bottom shell was studied. By a measurement in actual ship, it became clear that the initial deflection curve of unfairness was parabolic shape, and the initial bending stress existed in bottom plate. The critical hogging stress at which the skin stress of bottom plate reaches to yield stress was calculated for several unfairness. The allowable limit of unfairness was obtained for various frame space-plate thickness ratios.