This research aims to assess the impact of incorporating microcrystalline cellulose (MCC) into biocomposites. The biocomposite used uses an HDPE matrix with water hyacinth fiber reinforcement. The incorporation of microcrystalline cellulose (MCC) was conducted using volume fractions of 0, 1, 2, and 3%. Biocomposites are created using extrusion and hot pressing techniques. The research results showed that the incorporating of 1% MCC had optimal mechanical strength, where the known bending strength was 21.10 MPa with a bending modulus of 1129.14 MPa and an impact strength of 39.85 kJ/m2. An increase in crystallinity also occurred with the addition of 1% MCC which had a crystallinity of 59%. The increase in strength and bending modulus occurs because MCC can fill the pores and form a network, which is one factors that increases the flexural strength of the composite. Meanwhile, the incorporation of MCC exceeding 1% will result in a decrease in crystallinity to 58% for 2% MCC and 57% for 3% MCC. The same thing happened to the bending strength which decreased to 20.27 MPa for MCC 2% and 18.81 MPa for MCC 3%. The same thing also happened in the impact test, when MCC was added exceeding a volume fraction of 1%, the impact strength decreased to 34.65 kJ/m2 for 2% MCC and 33.27 kJ/m2 for 3% MCC. Adding too much MCC results in clumping, causing voids.