Well-defined functionalized sisal cellulose fibers (SCFs) grafted on hyperbranched liquid crystals (HLP) were synthesized to improve the compatibility between SCFs and epoxy resin (EP). The influence of SCFs-HLP on the mechanical and thermal properties of SCFs-HLP/EP composites was studied. The results show that the mechanical properties of SCFs-HLP/EP composites were enhanced distinctly. Particularly, compared with EP, impact strength, tensile strength, and flexural strength of composites with 4.0 wt % SCFs-HLP were 38.3 KJ·m−2, 86.2 MPa, and 150.7 MPa, increasing by 118.7%, 55.6%, and 89.6%, respectively. As well, the glass transition temperature of the composite material increased by 25 °C. It is hope that this work will inform ongoing efforts to exploit more efficient methods to overcome the poor natural fiber/polymer adhesion in the interface region.