The optimum design of a steel structure used to be the one having minimum weight. The structure tended to be thin-walled with various stiffeners. However, the construction environment has been changed: fabrication cost has increased, durability and maintenance has become more concerned and so on. Therefore, the sole consideration of the minimum weight may not be always a good guideline for the optimum design nowadays. The usage of thicker plates with less stiffeners can be a better choice for some structures. In fact, some design codes have provisions along this line. A so-called compact section is of that type, which has no stiffeners and yet plastic behavior prevails before buckling. Japanese design specifications for highway bridges do not have those provisions yet. In the present study, the three-dimensional nonlinear finite element analysis of a steel I-section member is conducted to find the width-to-thickness ratios of a flange and a web with which the cross section can attain plastic moment without undergoing buckling. The geometrical initial imperfection that influences the buckling strength conforms to the requirement of the fabrication error imposed by the Japanese design specifications for highway bridges in this analysis. The numerical results indicate some discrepancy from the width-to-thickness ratios of the compact sections specified by Eurocode 3. Also, the influence of the material behavior beyond the yield point, which is not defined by Japanese Industrial Standards (JIS), is discussed.