Kenaf fibers (KFs) were utilized as reinforcements to prepare poly(butylene succinate-co-adipate) (PBSA) green composites. Untreated KFs of KF35 (large dimension) and KF120 (small dimension) imparted the nucleation effect for PBSA crystallization. KF120 exhibited superior nucleation efficiency compared with KF35. The tensile/flexural moduli of PBSA drastically increased after the addition of KF35 or KF120. Successful modifications of KF35 through NaOH(aq) and 3-aminopropyltriethoxysilane (APS) were confirmed. The APS-treated KF (KF35AS) exhibited enhanced interaction with the PBSA matrix compared with NaOH-treated (KF35A) and untreated KF35. KF35A and KF35AS also facilitated the nucleation of PBSA crystallization. The KF3SAS-incorporated composites exhibited the highest tensile/flexural moduli among the different KF-added systems at identical loadings. The tensile and flexural moduli increased to 629% and 360% (40 wt % loading), respectively, compared with PBSA. The enhanced interfacial interaction between KF35AS and PBSA lessened the negative influence of KFs on the thermal stability and water absorption of PBSA.