In the current work, the fracture toughness of sand-particle- and wood-flake-reinforced polyester composites was studied under a linear elastic fracture mechanics approach. The effects of the particulate volume fraction (0–60 vol %) were studied. Scanning electron microscopy was used to observe the damage features on the composite surface. Results showed that sand-particle- and fine-wood-flake-reinforced polymer composites exhibited better results at 40 vol % than at other particulate volume fractions. Meanwhile, coarse-wood-flake-reinforced polymer composites showed higher properties at 30 vol % than at other particulate volume fractions. Observation of the composite surface after tests showed that sand particles have poor interfacial adhesion compared with wood flakes.