Nowadays, the production of cement is growing every year due to its higher consumption in the construction industry. On the other hand manufacturing of cement is environmentally unfriendly, and energy consumption and the price of cement are costly. Recently several studies have been carried out that focus on finding a possibility for alternative cementing materials. Industrial and agricultural wastes are among the possibilities. Wheat Straw is one of such agricultural residues which is available in Ethiopia but didn`t get much attention as alternative cementing material. This study aimed to investigate the effect of partial replacement of cement with wheat straw ash (WSA) in cement mortar at 0%, 5%, 10%, 15%, and 20% by volume of cement. In this study, a detailed experimental analysis was performed to evaluate the fresh, hardening, and microstructural properties of mortar. The workability of fresh mortar, compressive strength, water absorption, porosity test, Pulse velocity (UPV), and Sulphate attack, tests are conducted for different curing ages (3, 7, 28, 56, and 90 days). In addition, the morphology scanning electron microscopy (SEM) test, x-ray fluorescence (XRF), and BET surface area analysis test were conducted for WSA. And all physical tests for fine aggregates were established. The results revealed WSA10 replacement as the maximum compressive strength. Whereas WSA15 and WSA20 replacement has shown a slight reduction as compared to the control mix but the workability of mortar decreases with an increasing percentage of replacement due to the high surface area of WSA. Also, the results indicated that the mechanical performance of mortar at WSA10 indicated enhancement as shows more calcium silicate hydrate, thus reducing the porosity of the cement matrix. Due to the involvement of amorphous silica reacts with calcium hydrate and results in the formation of addational calcium silicate hydrate. Consequently, uniform and denser morphology was formed, and a good durability was enhanced as hydrated gel increased. Thus, it is recommended that the utilization of agricultural wastes such as WSA as supplementary cementitious material in mortar production should be encouraged. Keywords: Wheat Straw Ash (WSA), Fresh properties, Mechanical properties, Microstructure, durability