Crystallization kinetics and crystal structure of poly(L-lactide) (PLLA) and its blends with poly(D-lactide) (PDLA) and poly(ethylene glycol)-b-poly(D-lactide) (PEG-b-PDLA) have been investigated. Stereocomplex (SC) crystallites could be observed in both PLLA/PDLA and PLLA/PEG-b-PDLA blends. The flexible PEG chains promoted the formation of SC crystallites and the growth of PLLA homochiral (HC) crystallites. The isothermal crystallization involves the simultaneous growth of the two crystals as evidenced by differential scanning calorimetry and polarized optical microscopy, HC crystallites grow on SC crystallites and exhibit much slower crystallization rate at high crystallization temperatures. From atomic force microscopy and in situ X-ray scattering, the evolution of crystal structure and morphologies of HC and SC crystallites have revealed that there exists a competitive relationship between HC and SC crystallites and it will be affected by PEG chain segment.