A terahertz oscillator may include a first insulating layer, an electron emitter on the first insulating layer, adapted to emit an electron beam, and including a cathode, an anode, an oscillating circuit, and a collector sequentially disposed, spaced apart from each other, on the first insulating layer in a direction in which the electron beam is emitted from the electron emitter, wherein the oscillating circuit converts energy of the electron beam to energy of an electromagnetic wave, and wherein the collector collects the electron beam, an output unit adapted to emit the electromagnetic wave from the oscillating circuit to outside of the terahertz oscillator, and an electron emitting material layer. The cathode may include a first curved portion that extends in a direction perpendicular to the first insulating layer. The electron emitting material layer may be on an inner surface of the first curved portion of the cathode.