This invention relates to a high thermal conductivity composite material which comprises diamond particles and a copper matrix useful as electronic heat sinks for electronics parts, particularly for semiconductor lasers, high performance MPUs (micro-processing units), etc., also to a process for the production of the same and a heat sink using the same. According to the high thermal conductivity diamond sintered compact of the present invention, in particular, there can be provided a heat sink having a high thermal conductivity as well as matching of thermal expansions, most suitable for mounting a large sized and high thermal load semiconductor chip, for example, high output semiconductor lasers, high performance MPU, etc. Furthermore, the properties such as thermal conductivity and thermal expansion can freely be controlled, so it is possible to select the most suitable heat sink depending upon the features and designs of elements to be mounted.