In this article, the effect of alkaline peroxide bleaching treatment using hydrogen peroxide on mengkuang leaf fiber (MLF) to the tensile, thermal properties and water absorption of thermoplastic elastomer composites based on ethylene-vinyl acetate/natural rubber and MLF was investigated. Fiber morphology and the fiber-matrix interface were further characterized by scanning electron microscopy (SEM). Treated composites showed better tensile properties than untreated fiber composites. SEM of the fractured surfaces of the composite specimens indicates that the bleaching treatment improved the interfacial interaction between the fiber and the matrix. The results of thermogravimetric analysis showed a higher thermal stability for treated composites than untreated composites. Furthermore, alkaline bleaching treatment has resulted in reduction of water absorption from 31% to 26% as compared with untreated composites. J. VINYL ADDIT. TECHNOL., 26:127-134, 2020. FIG. 6. Scanning electron micrographs of MLF subjected to different treatments. (a) untreated MLF (mag 500×), (b) untreated MLF (mag 700×), (c) treated MLF (mag 500×), and (d) treated MLF (mag 700×).