Bio-based composites are getting popular due to its major contribution to the green materials industry application. Green techniques reduce the toxic gas emission, waste generation and improve environmental protection. The extraction of cellulose microfibers from natural fibers using the steam explosion technique is a cost-effective and traditional method. In this study, cellulose microfiber was extracted from raw Grewia optiva (GO) fiber by a chemo-mechanical method. Chemical treatments like alkaline treatment, steam explosion, bleaching, and acid treatment are used. Also, mechanical defibrillation of fiber was carried out by stirring and homogenization. The chemical compositions of various stages were determined by using ASTM standards. The microscopic, XRD, FTIR and TGA techniques were used to analyze structural and morphological properties of fibers. The extracted cellulose micro Grewia optiva fibers (CMGOF) had an average diameter of 10 μm. TGA results indicated that the thermal properties of isolated cellulose microfibers are improved after chemical treatment. The cellulose micro Grewia optiva fibers reinforced epoxy composites were fabricated adopting the compression molding method by varying the fiber ratio. Also, the mechanical properties of composites with various fiber ratios were investigated. The experimental results revealed that composite having 40 wt% of fiber has shown higher tensile, impact strength and hardness properties. © 2020 Taylor & Francis.