A stiffener shape optimization problem of thin plate built-up structures under compression is treated numerically. The stiffened plate structures are discretized into the isoparametric finite shell elements, and the sequential quadratic programming (SQP) method with a design sensitivity of buckling stress calculated by the finite difference scheme is applied to determine the optimal stiffener shapes which maximize a minimum bucking stress to which the volume is subjected. The optimal shapes of a unidirectional stiffener and a cross stiffener under uniaxial compression are calculated by the iteration of the SQP. From the results, it is recognized that the optimal stiffener shapes of the thin plates are closely related to the buckling modes.