The objective of this research is to present formulation and solution methodology for optimum design of thin-walled composite beams. The geometric parameters and the fiber orientation of beams are treated as design variables simultaneously. The objective function of optimization problem is to maximize the critical flexural-torsional buckling loads of axially loaded beams which are calculated by a displacement-based one-dimensional finite element model. The analysis of beam is based on the classical laminated beam theory and applied for arbitrary laminate stacking sequence configuration. A micro genetic algorithm (micro-GA) is employed as a tool for obtaining optimal solutions. It offers faster convergence to the optimal results with smaller number of populations than the conventional GA. Several types of lay-up schemes as well as different beam lengths and boundary conditions are investigated in optimization problems of I-section composite beams. Obtained numerical results show more sensitivity of geometric parameters on the critical flexural-torsional buckling loads than that of fiber angle.