The global trend of environmental protection has led to replacing synthetic fibers in every feasible application with natural fibers. An alteration of the fiber's surface is essential for the natural fiber to be more suitable for use. In this study, the mercerization processes including 5% (wt/vol) sodium hydroxide solution at five different soaking times (15, 30, 45, 60, and 75 min), were used in this study to enhance various features of Ziziphus nummularia fibers isolated from Ziziphus nummularia plant stems. Results showed that 60 min alkalized Ziziphus nummularia fibers had cellulose content of 65.72%, which seems to be higher, and it was termed as optimally alkalized Ziziphus nummularia fibers. The thermal studies showed that optimally alkalized Ziziphus nummularia fibers had a degradation temperature of 360 C. The optimally alkalized Ziziphus nummularia fibers showed a crystalline index of 50.6% and crystal size of 3.52 nm. Epoxy composites were manufactured with various loading percentages of optimally alkalized and raw Ziziphus nummularia fibers to prove the application suitability. It was shown that 20 wt% of optimally alkalized Ziziphus nummularia fibers-based epoxy composites showed a higher ultimate tensile strength of 40.31 MPa than all other composites. Scanning Electron microscope studies elucidated the fiber's surface behavior and other characteristics of fibers and tested epoxy composites.