This research work aims to find the tensile stress and the tensile modulus of cellulose microfibril (CMF) reinforced epoxy polymer composites. CMFs were extracted from banana fibers using the acid-alkali treatment. Scanning electron microscopy (SEM) images revealed the isolation of α-cellulose of 1-2 μm after the acid-alkali treatment. Thermogravimetric analysis showed an increase in the thermal resistance of CMFs up to 380°C, compared to 260 °C for untreated natural fiber. The influence of three parameters, namely NaOH% (w/w), fiber diameter (μm) and fiber volume% (w/w), on the tensile behavior of composites was investigated. Response surface methodology (RSM), a DOE tool, was used for determining the composition of different specimens. A three-factor, three-level Box-Behnken Design (BBD) model of RSM was selected for investigating the effect of input variables on the tensile behaviors and to obtain the optimization conditions. By solving the regression equations and analyzing the 3D response surface plots obtained from BBD, the optimized desirability ramp values were obtained. The optimized tensile stress and tensile modulus values were 41.78 MPa and 10380 MPa, respectively, using optimized values of NaOH% (w/w), fiber diameter (μm) and fiber volume% (w/w) of 18.32, 250 and 4.05 respectively. © 2020 Editura Academiei Romane. All rights reserved.