Effect of lysine triisocyanate (LTI) as an additive on the microstructure and the mechanical properties of biodegradable polymer blends of poly(L-lactic acid) (PLLA) and poly(butylene succinate-co-epsilon-caprolactone) (PBSC) were investigated by using Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), field-emission scanning electron microscope (FE-SEM), and bending and mode I fracture testing techniques. It was found that the addition of LTI dramatically improves the phase-separation morphology of the PLLA/PBSC blends. FTIR analysis suggested that the NCO groups of LTI were acted as compatibilizer by attributing secondary process between the two polymers PLLA and PBSC, resulting lower peak intensity of ACI samples, understood as secondary process is polar interaction and hydrophobic process. DSC and FE-SEM results also supported such improvement of immiscibility between PLLA and PBSC. Macroscopic mechanical properties such as the bending fracture energy and the mode I fracture properties are also effectively improved by the LTI addition. Void formation is suppressed due to such morphological change in PLLA/PBSC/LTI blends, and, as a result, energy dissipation in the notch-tip region of the blends becomes higher than in that of PLLA/PBSC where localized stress concentration due to voids tends to accelerate fracture initiation and, therefore, lowers the fracture energy. POLYM. ENG. SCI., 50:1485-1491, 2010. (C) 2010 Society of Plastics Engineers