The continuous increase of power density in electronic devices requires advanced materials with high thermal conductivity combined with a reduced coefficient of thermal expansion (CTE). For application of diamond based composites as heat sink or electronic housings in mobile applications an additional requirement is low weight. The combination of a light weight matrix material, e.g. aluminum with diamond as a filler having a high thermal conductivity but low CTE results in a material with thermal conductivities in the range of 300 to 600 W/mK combined with reduced coefficient of thermal expansion. Usually aluminum-diamond composites are reported to be prepared in literature by infiltration processes (squeeze casting or gas pressure infiltration). A PM approach was used to prepare aluminium-diamond composites. Large plates with size of 150mm×150mm were prepared and characterised with respect to the thermal diffusivity/conductivity by using a local thermal mapping technique. In addition first results of thermal analysis from PM prepared Mg-diamond composites are reported.