The molecular weight of poly(d-lactide) (PDLA) and poly(l-lactide) (PLLA) affects the performance of their blends by controlling the formation and characteristics of stereocomplex (SC) crystals. In this work, the crystallization behaviors and rheological properties of asymmetric PDLA/PLLA (5/95, w/w) blends were carefully investigated considering the molecular weight of PDLA (Mn_(PDLA) = 1.5 × 10³–18.7 × 10³ g/mol) when the molecular weight of PLLA was selected as 5.0 × 10⁴ g/mol. The results revealed that PDLA holding too low or too high Mn_(PDLA) was unfavorable to the formation of SC crystallites owing to their weak interaction or difficult diffusion in the PLLA matrix, and the optimal Mn_(PDLA) was ca.5.4 × 10³ g/mol. Given this optimal Mn_(PDLA), the composite viscosity and tensile viscosity of the obtained PDLA/PLLA blend were increased by 2–3 orders of magnitude and 10 times compared with neat PLLA, respectively, due to the formation of SC crystals with a high content (ΔH_m-SC of 26.0 J/g). Besides, this PDLA/PLLA blend showed a fast crystallization rate and strong solid-like viscoelastic behavior. This work revealed the regular correspondence between Mn_(PDLA) and the performance of asymmetric PDLA/PLLA blends.