In the present work, a new class of polymer composite is developed using walnut shell powder (WSP) as particulate filler in the epoxy matrix. Three different sizes of WSP are used for preparing three different sets of composites. The particle size selected is 50-micron, 75-micron, and 100-micron. In each set of composites with different particle size, four compositions i.e., 5 wt. %, 10 wt. %, 15 wt. % and 20 wt. % of the WSP is fabricated. All prepared composites have undergone testing to study the behavior of the material under mechanical loading. The different mechanical tests performed are the tensile test, flexural test, compressive test and hardness test. From the experimentation, the inclusion of WSP appreciably alters the different mechanical properties of epoxy. With the increase in the content of filler, an appreciable increase in the value of hardness and compressive strength is observed. Though, it has been seen that the tensile strength and flexural strength of the material is compromised slightly when the content of WSP increases beyond a certain limit. Further, it is observed that composites fabricated with smaller size particles give improved mechanical properties as compared to their counterparts.