For toughening thermoplastics by using elastomeric components, the relatively high contents of elastomeric phase are commonly demanded to trigger the brittle-to-ductile transition. However, an obvious drawback of remarkably decreased strength and rigidity may arise after incorporation of large amount of elastomeric species. The main thinking in our present work is to achieve a good toughness-strength balance in an elastomer-toughened thermoplastic system with less amount of elastomeric phase, i.e., the blend of poly(lactic acid) (PLA)/olefin block copolymer (OBC) 90/10 w/w. When both of the thermodynamics favorable compatibility and the thermodynamically stable morphology were realized, the impact toughness of PLA/OBC 90/10 blend was 25 multiples for that of pure PLA, while the tensile strength could preserve as a level of 87 % (based on the value of pure PLA). The interfacial compatibilization between PLA and OBC achieved through adding EMA-GMA as acompatibilizer. On the other hand, a quiescent annealing process at 90 ºC resulted in a more thermodynamically stable morphology, which was characterized as high crystallinity, large size of elastomer-phase droplet, and thickening interfacial layer. Our study offers new insight into the optimization of properties of multicomponent blend that except for realizing the thermodynamics favorable compatibility, the transition from kinetics-dominated morphology to thermodynamically stable one also plays a crucial role.