Abstract: A flexible explosive is developed in the present paper using explosive PETN as the base explosive and methyl silicone rubber as a bonder, which is cured at room temperature. The flexible explosive is appropriate for use in fine calibred charges because of its small critical diameter and weak diameter effect. Processes in its development, including the technique and principle of explosive mixing, are described. Chief factors affecting the mixing technique are analyzed. The critical cross section or critical diameter and diameter effect, i.e., D - 1/S, of the detonation propagation in a square channel of the flexible explosive have been experimentally measured.