This study investigates the effect of rubber viscosity on the changes in crystallization properties, blend morphology and impact resistance of PLA/NR blends. Here, natural rubber grade STR5L was masticated in an internal mixer for 15 minutes at various mixing speeds, which were 20, 40, 60, 80, and 100 rpm, respectively. It was found that mastication of natural rubber led to disappearance of gels and reduction in molecular weight of natural rubber. After mixing masticated and unmasticated NRs with PLA at 10 % NR content, all the blends showed higher impact resistance than neat PLA, especially the blend masticated at 20 rpm showed the highest impact resistance. Moreover, we found that the blend with short crystallization time also provided high impact resistance with small rubber-phase size.