For the purpose of finding the effect of biaxial imperfection on column strength, a series of tests on H-columns with initial deflections and eccentricities under compression combined with biaxial bending were carried out. The standard test conditions of total 28 columns are that the end sections are biaxially simply supported with warping constrained and that columns have no residual stresses before tests. Only two of them are specimens with residual stresses and the end sections of another two are simply supported with respect to weak axis bending but clamped about strong axis bending. The elastic plastic behaviors of columns tested were numerically analyzed with use of an incremental stiffeness method. The ultimate strength of columns with imperfections were also calculated from an approximate strength formula with consideration of inelastic stability. Reasonable agreement was found between the observed column strength and both calculated values. Two kinds of imperfection, eccentricity and initial deflection, have so similar effects on column strength that the difference between them seems to be negligible for designers. Such a middle-length column that its elastic buckling load equals to its fully plastic axial load is the most sensitive to imperfections. Therefore, such a column is considered as a critical one in order to determine the allowable values of imperfections.