PURPOSE: To obtain good quality diamond crystal particles having a predetermined narrow particle size distribution by bringing metal catalyst into contact with non- diamond carbon and raising the press. in two steps in the temp. range from the lowest limit of the catalyst-carbon reaction temps. to a specific temp.

CONSTITUTION: Metal catalyst such as pure Co disc and non-diamond carbon such as artificial graphite disc are alternately superposed, housed in a reactor, and brought into contact with each other at high temp. and press. At this time, the temp. is set to above the lowest limit of the catalyst-carbon reaction temp. and below a temp. about 25% higher than the lowest limit temp. In the temp. range, the reactor is first held for above 0.5 min at an arbitrary temp. selected from the range and a press. within about 4% higher than the lowest limit of pressures at which diamond is formed when the temp. is maintained. Next the press. is raised to the range above 4% higher than said lowest limit and below about 15% higher than the limit, and held for about 10W15 min. Thus, diamond particles more than 90% of which have a particle size above 200μm are obtained.