The continuous progress of the electronic industries put forward a new requirement to the electronic components that must have an excellent heat conduction performance. Thus diamond-Cu composite is developed as a high thermal conductivity and low coefficient of thermal expansion material. A vacuum hot pressing method is chosen to prepare diamond-Cu composites and the thermal conductivity of the diamond-Cu composite is researched. The effects of different contents of chromium, the size of diamond particles and the content of diamond particles on the thermal conductivity of the diamond-Cu composite are discussed. The results demonstrate that the chromium element can improve the thermal conductivity of the composites and the thermal conductivity is largest when the content of chromium is 3 percent.