Abstract: To prepare the diamond/copper composite materials with good thermal and physical properties, the surface of diamond particles was modified by chromium plating, and the composites were prepared by vacuum hot pressed sintering (VHP). The microstructure and fracture morphology of the diamond/copper composites were analyzed, the bending performance, thermal conductivity, and thermal expansion coefficient of the composites were tested, and the optimum process parameters of the composites prepared by VHP were studied. The results show that the chromium plating on the diamond surface can improve the interface bonding of the composites. The flexural strength of the composites increases with the increase of sintering temperature, and the flexural strength of the composites prepared at 1100 ℃ reaches 278 MPa; when the other conditions are the same, the thermal conductivity of the composites is 417 W∙m⁻¹∙K⁻¹, while the thermal conductivity of the composites without chromium plating is only 127 W∙m⁻¹∙K⁻¹. As the sintering temperature increases, the thermal conductivity of the composites increases. When the sintering temperature is 1050 ℃ and the pressure is 20 MPa, the thermal conductivity of the chrome-plated diamond/copper composite can reach 574 W∙m⁻¹∙K⁻¹. Within the temperature range of 25 to 300 ℃, the thermal expansion coefficient is between (4.90~8.50)×10⁻⁶ K⁻¹, and the composite materials exhibit the excellent low thermal expansion properties with the increase of test temperature.