This work presents structural optimization studies of aluminum and composite material horizontal tail plane of a helicopter by using MSC. NASTRAN SOL200 optimization capabilities. Structural design process starts from conceptual design phase, and structural layout design is performed by using CATIA. In the preliminary design phase, study focuses on the minimum weight optimization with multiple design variables and similar constraints for both materials. Aerodynamic load calculation is performed using ANSYS and the finite element model of the horizontal tail plane is created by using MSC.PATRAN. According to the characteristics of materials, design variables are chosen. For the aluminum horizontal tail, thickness and flange areas are used as the design variables; and for composite horizontal tail, attention is mainly focused on the ply numbers and ply orientation angles of the laminated composite panels. By considering the manufacturability issues, discrete design variables are used. For three different mesh densities, different initial values of the design variables, and similar design constraints, optimizations are repeated and the results of optimizations are examined and compared with each other. In the optimizations performed, constraints are taken as strength and local buckling constraints. It is shown that the optimization methodology used in this study gives confident results for optimizing structures in the preliminary design phase.