Biomaterials include a broad class of materials that are used in medicine and dentistry such as: metallic biomaterials, ceramic and glass biomaterials, non-degradable synthetic polymers, biodegradable polymers, bio-derivative polymers as well as passive and bioactive coatings [1]. Ultra-high molecular weight polyethylene (UHMWPE) is one of the promising matrix materials due to its high chemical inertness, the highest wear resistance of any thermoplastics, low friction coefficient and bioinertness make it the material of choice for hip joints and knee joints. Other excellent properties, such as low density, high strength, high modulus, good abrasion, and chemicals resistance [2,3]. To further improve its mechanical properties and tribological response, fillers/reinforcements are incorporated into the polymer [4]. For biomedical applications, the HAp is very interesting filler because is one of the bioceramics with good bioactivity and is a major mineral constituent of the bone matrix [5]. In this study, HAp reinforcement into UHMWPE matrix and its effect on mechanical properties of the UHMWPE composites were investigated. UHMWPE composites reinforced with 0.5, 1 and 2.0% wt., respectively nano HAp powders were successfully produced by solution and gelation method. SEM studies showed that HAp nano particles were homogenously distributed into UHMWPE matrix and good cross-linked with the matrix. FTIR results revealed that HAp incorporation into matrix was conducted and crystallization of UHMWPE increased by increment in amount of HAp results in deepening crystallization peaks at nearby 500 and 1500 cm-1. DSC results, which is useful technique to determine the variation of melting point and crystallization ratio of UHMWPE composites, indicated that there was no remarkable change in melting points of composites, while crystallinity of the samples generally showed slight increase by increasing amount of nano HAp particles. The tensile test instrument was utilized to determine elastic modulus of the samples and their elastic modulus were raised from 1050 MPa (pure UHMWPE) to 1900 MPa with higher HAp reinforcement. It can be concluded that UHMWPE-2 % wt. HAp composites have promising results by combining of crystallinity and elastic modulus.