This invention is to provide a method for the production of diamond at a high rate and in a high efficiency using in-liquid plasma. The present invention is a method for the production of diamond using electromagnetic waves irradiated to a liquid containing carbon, hydrogen and oxygen in which the ratio of hydrogen atoms to the sum of carbon atoms and hydrogen atoms is from 0.75 to 0.82 and the ratio of carbon atoms to the sum of carbon atoms and oxygen atoms is from 0.47 to 0.58 so as to generate plasma in the liquid.