A die for an underwater cutting type pelletizer, the die including a die orifice part in which a thermoplastic resin is inhibited from solidifying within the die orifices. The die can have a reduced difference in temperature between the die orifice part and an inner periphery-side fixing part for fixing the die orifice part to a mixer/extruder, and can thereby inhibit the generation of thermal stress. The die is characterized by being equipped with a heating jacket for heating the inner periphery-side fixing part, the heating jacket being disposed within the inner periphery-side fixing part along an inner annular passage formed within either the die orifice part or the inner periphery-side fixing part or both along the boundary between the die orifice part and the inner periphery-side fixing part.