The effect of initial imperfection on the buckling strength of a straight pipe under bending has been investigated by nonlinear FEA, considering the effect of a cross sectional oval deformation by changing the variables of pipes, that is, L/D varying from about 2.5 to 20 and D/t varying from about 50 to 200. Furthermore also the buckling strength of curved pipes (D/t = 200) have been investigated by changing R/D from 50 to 200. Not only the elastic buckling but also the elasto-plastic buckling was investigated. From the numerical results, the followings were found. The buckling strength reduced more in longer pipes than shorter pipes due to pre-buckling oval deformation. But, the reduction of buckling strength due to imperfection is larger in shorter pipes than in longer pipes. Moreover, the buckling strength reduces more by the imperfection of buckling mode than by that of oval mode. The effect of imperfection on buckling strength in plastic region was smaller than in elastic region. The buckling moment of curved pipes reduces more when the curvature of pipe increases in both original and imperfect pipes, especially for longer pipes.