Diamond has excellent physical and chemical properties. So diamond was applied to some industrial products. But natural dia-mond and high pressure and high temperature synthesized diamond have limited to their application by their forms and sizes. Though diamond particles and films can be obtained using CVD methods, there are many problems such as low adhesion strength to use as tools and mechanical parts. In this paper, Diamond growth state using microwave plasma CVD and adhesion strength between diamond film and WC-Co substrate were studied. Cutting test of A1-18mass%Si (A390-T6) alloys using diamond coated tool was carried out. As a result, low temperature growth was recognized on the scratched after Co removal WC-Co substrates. On the cutting test, using the tool which coated diamond at high temperature (about 1173K) after Co removal and scratch pre-treatment, cutting length was less than 200m and crater wear was severe. Using the tools which coated diamond at low temperatures (from 680K to 779K) after Co removal and scratch pretreatment, following increase of substrate temperature, cutting length became being long and crater wear became being mild. Compared with the tool which coated diamond at high temperature after Co removal and scratch pretreatment, the tools which coated diamond at low temperature after same pre-treatment improved cutting performance. Especially the tool which coated diamond at 779K performed more than ten times longer life than the tool which coated at high temperature after same pre-treatment. When suitable pre-treatment and growth conditions of basic factors on diamond synthesis were selected, diamond coated tools which have good cutting performance could be made.