The purpose of the study is to determine the optimal condition for mixing factors of natural cellulose or epoxy composite by using response surface method. Three mixing factors selected for the study were degassing agent content, stirring time, and sonication time. As a response, the fracture toughness of the composites was determined by using single edge notch bending test. The level of each factor was selected at 3 conditions which were low, medium, and high. The analysis of variance showed degassing agent content and sonication time were the most significant factors affecting the fracture toughness of the composite. The optimal conditions for mixing factors were 1.34 wt% degassing agent content, 5 minutes for stirring time, and 60 minutes for sonication time. The predicted fracture toughness was 1.6836 MPa.m0.5 with a desirability value of one. Three validation experiments were conducted to test the generated mathematical model and results showed the percentage difference was in the range of 6% to 15%.