Main properties of magneto-optic modulators with single crystal films of garnets, the basic principle and the experimental installation of the measurement of Faraday rotation angle θ by means of the frequency doubling method of magneto-optic modulation are described. The means and merits of the measurement are presented, the results measured are discussed, and the measuring error is analysed. The Faraday rotation angle θ measured with the new moans is not effected by the light intensity change of a laser source and rnistable factors of other devices in a optical circuit, so that the measuring precision is ±0,020°.