TO DETERMINE THE OPTIMUM TOOL COATED WITH DIAMOND FILM, A SERIES OF EVALUATION TESTS WERE PERFORMED ON EXPERIMENTAL TOOLS PRODUCED BY VARIOUS SYNTHESIZING CONDITIONS AND SUBSTRATE TOOL MATERIALS. FROM THESE TESTS, SILICON NITRIDE CERAMIC TOOL WAS FOUND TO BE ONE OF THE BEST SUBSTRATE MATERIALS FOR DIAMOND COATED TOOL. IN MACHINING OF LOW SI CONTENT ALUMINUM ALLOY AND FRP, THE DIAMOND COATED SILICON NITRIDE TOOL SHOWED ALMOST THE SAME TOOL WEAR AS THAT OF SINTERED DIAMOND TOOL. IN PARTICULAR, FOR LOW SI CONTENT ALUMINUM ALLOY, IT WAS ABLE TO CUT AT A CUTTING SPEED ABOVE 1500 M/MIN.