This study aimed to investigate the effects of blend compositions on packaging related properties of poly(lactic acid) (PLA) and low density polyethylene (LDPE) blown films. Blend films with PLA contents of 5–20 wt% were produced and compared. SEM micrographs of cross-sectional cryofractured surfaces of the blend films revealed that in situ fiber-reinforced composites were obtained. Viscosity ratio of the polymer components of ~ 1 confirmed that fiber formation was favorable for this blend system. PLA microdomains dispersed throughout the film in forms of long fiber (L/D > 100) and ribbon. Number of fibers and ribbons increased with an increase of PLA content. Critical content of PLA was found to be 20 wt% to effectively improve both moduli and gas barrier properties. Incorporation of PE-co-MA compatibilizer showed minimal effect on PLA structure. It, however, did improve moduli and O2 barrier properties when sufficient amount of 1.5 pph was used in 10 wt% PLA/LDPE. In short, flow behavior, ratio of the polymer components, and degree of compatibility together played intricate roles on the morphology and hence mechanical and transport properties of the PLA/LDPE immiscible blend.