By using the highly oriented 1:1 stoichiometric blend of high-molecular-weight PLLA and middle-molecular-weight PDLA species and by choosing a suitable annealing condition, we have succeeded for the first time to observe the phase transition phenomenon in the heating and cooling processes by measuring the temperature dependence of the X-ray fiber diagram and polarized IR spectra. The as-drawn sample was found to consist of the two crystalline phases; disordered alpha form (alpha') and stereocomplex (beta(c)). In the heating process, the alpha' form was found to be reorganized into the oriented alpha phase above 120 degrees C, which melted around 180 degrees C. However, by heating this sample some more, some parts of the molten chains are found to crystallize on the surface of the beta(c) crystallites with high degree of chain orientation. In the subsequent cooling process from the beta(c) phase, the unoriented alpha crystal appeared at around 120 degrees C and coexisted with the highly oriented beta(c) crystals.