The use of petroleum-based polymers is very harmful to the environment and is non-renewable. An alternative to replace the material is to create a polymer-based composite with natural fibers. Natural fibers are an environmentally friendly and renewable basic material. Natural fiber composites have several advantages including the material is recyclable, cheap, low density, and strong. The strength of the composite is affected by the matrix, the fibers, the distribution of the fibers in the matrix and the interface bonds between the fibers. Natural fibers have a high surface energy, so they tend to clump at a single point. The addition of dispersion agent to the fiber is intended to improve the distribution of fibers in the composite matrix. MCC is used to improve the interface bond between fibers and matrices, but MCC has the property of clumping easily, so temperature treatment is carried out to reduce the moisture of MCC and make better distribution on the composite. This study aims to examine the effect of the addition of dispersion agent and MCC on the mechanical properties of HDPE composites with water hyacinth fiber reinforcement. This study used variations in the addition of dispersion agents of 0.5%, 0,99 %, and 1,96 %. Meanwhile, the temperature treatment of MCC uses variations of 60°C, 80°C, and 100°C.  The composites are made using dry mixing, extrusion and hot pressing processes. The results of the study for dispersion agent variations were obtained that the addition of dispersion agent to HDPE composites with water hyacinth fiber reinforcement had an effect on the tensile strength, bending and impact. The optimum point of mechanical strength of hyacinth-HDPE fiber composite was obtained in the variation of 0.5% dispersion agent The tensile and bending strength was achieved in these variations of 13.32 MPa, 18.89 MPa. The highest impact strength occurred in the variation in the addition of dispersion agent 0,99 % with a value of 37.48 kJ/m2. Meanwhile, the addition of MCC with temperature treatment on the mechanical properties of HDPE composite with water hyacinth fiber reinforcement has an effect on tensile strength, bending and impact. The optimal points of tensile strength and bending in hyacinth-HDPE fiber composite were obtained in the variation of MCC addition with 100°C temperature treatment with values of 14.22 MPa and 19.87 MPa. The highest impact strength of the addition of MCC with temperature treatment occurred at 80°C with a value of 37.75 kJ/m2.