A novel fiber optical interference method which called three-fiber Mach-Zehnder interferomety has been developed and demonst rated. The Mach-Zehnder interference patterns are affected by the special relative positions of the fibers and the polarization of the output optical fields. A helium-neon laser and three HiBi fibers are used in our experiments. The optical field intensity dist ribution function of the interference patterns is analyzed and the simulation results compared with experimental interference patterns are given.