Well-defined polymethylene/poly(D,L-lactide) diblock copolymers (PM-b-PLA) were synthesized via the combination of living polymerization of ylides and ring opening polymerization of D,L-lactide. The 1H NMR spectra, GPC traces and FT-IR spectra of diblock copolymers indicated the successful extension of PLA segment. The melting behavior of PM-b-PLA diblock copolymer was investigated by differential scanning calorimetry (DSC). The results indicated that the incorporation of PLA segments effectively influence the crystalline process of PM segments. Scanning electron microscopy (SEM) observation showed that PM-b-PLA can act as an efficient compatibilizer for LDPE/PLA blends. PM-b-PLA diblock copolymers can be fabricated to ordered porous films via the breath-figure (BF) method.