As a general trend, the size of oil tankers is growing larger for economical reason. As a result, there have been arrived many technical problems involved in design of super tankers which must be solved. One of such problems is the local strength, and especially the buckling failure of web plates with slots for longitudinals of transverse rings is one of the most important items to study. In fact, in 1965, many super tankers suffered from damages of buckling failures. A most serious buckling failure occured at web plates with slots for longitudinals of transverse rings. These failures were recognized at tank tests or official trials. Then, ship designers increased the thickness of web plates and added suitable stiffeners or collar plates. Although actual failure of this kind has not been reported recently in Japan, there is still no sufficient support of theoretical investigation, except those by experiments. For theoretical analysis on buckling of web plates with slots, the authors developed a particular computer program based on the finite element method using triangular finite elements of conforming shape function. We also conducted several kinds of experiments and compared the results with those of the theoretical analysis. A quite good agreement was observed between them.