The investigation of a magneto-optic isolator for fiber communication is reported. The isolator consists of a Faraday rotator made of rare-earth iron-garnet single crystal and two calcite prisms. Insertion loss of 2.4 db and isolation ratio of 34.8 db were obtained at 1.15 m wavelength. The improvement of isolator properties by using some rare-earth substituted iron-garnets and polarization-independent techniques are also discussed.