The mechanical measurement between fiber and medium in composites is an attractive bub difficult subject. We use a single-mode optical fiber instead of ordinary fiber to be burried in polyester. When He-Ne laser light goes through the single optical fiber and reference fiber with the same length, we could test the stress and strain in the optical fiber in composites according to the two-beam interference. Many results could be obtained about the interface miorocosmio mechanics in composites. This paper presents a new sensitive testing method for composites.