This paper describes results of shock recovery compaction experiments with a diamond / silicon composite powder. Under diamond compaction process, the addition of silicon causes to increase the hardness of recovered specimens. Because additive silicon reacts with diamond accompanying heat generation. This heat process contributes to reduction of micro and macro clack generation. The purpose of this study is to clarify the microstructure in the diamond compacts including silicon, and the effect caused by the addition method of silicon. By transmission electron microscopy (TEM) analysis, the boundary between diamond grains consist of amorphous carbon, graphite, silicon carbide and silicon. Silicon-carbite exist in the amorphous carbon layer as very fine particles.