Abstract: Diatomite was pretreated by calcination and acidification, surface-modified with three different silane coupling agents (KH550, KH590, Si69), and then used for the reinforcement of high-density polyethylene (PE-HD) through melt blending. The effects of the diatomite loading and coupling?agent type on the mechanical properties and thermal resistance of the composites were investigated. The compatibility and morphology of PE-HD composites with the KH590-modified diatomite were studied by Fourier transformation infrared spectroscopy and scanning electron microscopy. The results indicated that the KH590-modified diatomite was homogeneously dispersed in the PE?HD matrix, and the impact fracture surface exhibited a more uniform morphology due to the enhancement of the interfacial compatibility between diatomite and PE-HD. The optimum modification effect was obtained for the composites when 1 parts per hundred (phr) of KH590 and 2 phr of diatomite were incorporated. In this case, the tensile strength of the composites increased by 23.8 MPa, the impact strength increased by 122.1 kJ/m2, and the thermal decomposition increased by 10 °C.