Poly(l-lactic acid) (PLLA) and Poly(d-lactic acid) (PDLA) with different optical purity were blended in an internal mixer and their crystallization behavior were investigated. Nearly complete stereocomplex crystallites could be obtained at specific condition between PLLA and PDLA. The melting temperature of stereocomplex was related with the optical purity of PLLA and PDLA, and also with the mixing temperature and time. When the mixing temperature was set dozens of degrees above the melting temperature of pure PLLA and PDLA at a weight ratio 1:1, the stereocomplex could be formed within a short period of time, and no homocrystallization or trace homocrystallization in some samples could be found. The possible mechanism of stereocomplex formation was achieved in this article and the shear force play an important role during the melt mixing process.