In this study, oil palm fruit bunch fiber (OPEFB) was used as a secondary filler in HDPE/clay nanocomposites. The composites were prepared by melt compounding, containing high density polyethylene (HDPE), OPEFB fibers, Maleic anhydride grafted polyethylene (MAPE) and four different clay loading (3, 5, 7 and 10 PE nanoclaymasterbatch pellets per hundred HDPE pellets). Four OPEFB sizes (180 µm, 250 µm, 300 µm and 355 µm) were added in the composites to investigate its effects on the fracture toughness and impact strength. Fracture toughness of the composites was determined according to ASTM D5045 and single edge notch bending (SENB) was employed during the test while impact tests were performed according to ASTM D256. The effects of alkali treatment were also investigated in this study. The result indicates that the fracture toughness slightly increased as clay loading increased. The highest value of fracture toughness was 0.47 and 1.06 MPa.m1/2 at 5 phr for both types of composites. The presence of OPEFB fiber as a secondary filler in the matrix indicates significant enhancement of fracture toughness up to 133%. However, its impact strength seems to deteriorate with the presence of OPEFB fiber.