We prepared banana fiber (BF)/poly(butylene succinate) (PBS) composites using hexamethylene diisocyanate (HDI) as a compatibilizer. BF was treated with NaOH aqueous solution (BF-A). Both BF/PBS/HDI and BF-A/PBS/HDI composites were made by applying a batch type twin-kneader at 130°C. Dumbbell specimens were prepared by an injection molding machine at 130°C. Effect of concentration of NaOH aq on tensile strength and impact strength, effect of amount of HDI on tensile strength and Izod impact strength, and effect of mixing time on tensile strength and Izod impact strength of BF/PBS/HDI and BF-A/PBS/HDI composites were studied. When contents of HDI in composites were increased, tensile strength increased. In addition, when mixing times were increased, tensile strength increased. Although the mechanical properties of PBS were first degraded with the addition of BF, alkaline treatment to BF and the addition of HDI improved the mechanical properties. This probably due to the promotion of the adhesion between BF and PBS. The surface morphology of the fracture surfaces of the composites were observed with a scanning electron microscope (SEM). It shows that higher mechanical properties can be expected from compatibilized composites based on BF-As with aliphatic polyesters.