High Density Polyethylene (HDPE) composites were prepared and developed by injection moulding method with mercerized and acetylated Combretum Dolichopetalum Fiber (CDF) at variable weight fraction of fiber (2.5 -7.5 %) and HDPE matrix (92.5 -97.5 %). This research work is aimed at optimizing the mechanical properties of CDF reinforced HDPE composites. The mechanical properties (tensile strength, flexural strength, impact strength and hardness) were optimally determined using developed quadratic polynomial equation of response surface methodology (RSM) with three level factorial design (3 - LFD) and analysis of variance. Under optimum production of the composites, untreated C. dolichopetalum fiber increased the tensile strength (31.429 MPa) and hardness (24.5 HR) with reduced flexural (19.256 MPa) and impact strength (256.7 kPa) while mercerized CDF improved the tensile strength, flexural strength, hardness and impact strength by 2.51, 28.98, 51.02 and 17.84 % of untreated composite, respectively. Acetylated CDF reduced the tensile and flexural strengths with improved the hardness and impact strength by 52.38 and 38.15% of untreated fiber - HDPE composites, respectively.