Bio-plastics are widely used in dishes, packaging containers, automotive parts, and electronic products. To expand bio-plastic usage, improvements in impact strength, heat resistance, and flame resistance are required. As a method of improving the impact strength of plastics, alloying a matrix polymer with a compatibilizer and soft materials such as rubber has been studied. In contrast, elastic modulus reduction due to the rubber component is pointed out. Here, we introduce the impact strength improvement of bio-plastic without reduction of the flexural modulus. The impact strength of polyamide 11 (PA11) is 2.5 times (32.8 kJ/m2) that of neat PA11 (12.9 kJ/m2) by dispersing 10-30 wt% poly(butylene succinate) (PBS) without adding a compatibilizer and soft materials. The PA11/PBS alloy has a sea-island structure with a PA11 sea and PBS islands. Gaps of several tens of nanometers are observed at the PA11-PBS interfaces, and the sea and islands are connected by string-shaped bridge phases of 10 nm in diameter. The formation of dilational stress fields in the PA11 are suggested because tensile stress due to shrinkage of PBS particle is transmitted to matrix PA11 through these bridge phases during the cooling of the melt compounded PA11/PBS alloy.