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The letter presents a theory of the spectrum-narrowing effect of semiconductor lasers by optical feedback. A simple formula for the 3 dB spectral width is derived as a function of the intensity and phase of the returning beam and the round-trip time of the feedback circuit. The spectrum-narrowing ratio of a GaAlAs laser with a small amount of optical feedback was measured as a function of the optical feedback power ratio. The derived formula and experiment show very good agreement.