By analyzing the relation between input sequences and feedback polynomials, we find the reason why low-weight codewords for Turbo code are generated.The concept of optimal period interleavers for Turbo codes is introduced. A novel design scheme for interleavers used for Turbo codes is presented. The main idea of this method is to design the interleaver based on an optimal period interleaver. Simulation results are presented for Turbo codes using our interleavers. A magnitude improvement in bit error rate performance is obtained at high signal-noise ratio compared to Turbo codes using other conventional interleavers. According to simulations results, it is significant to apply the optimal period interleaver in the practical interleaver design. This is particularly of interest to Turbo coding applications in mobile communications where short frames are typical.