A fiber heterodyne interferometer probe free from external disturbance, which can measure small displacement of an object as a pressure transducer or a vibrational object, is described. The phase fluctuation in the fiber due to external disturbance is excluded using a reference beat signal of light beams from a reflection mirror on a graded index rod lens. A graded index‐type multimode fiber connected to one end of a single‐mode fiber by arc fusion splicing is used to improve acceptance of the light. The characteristics of the fiber probe are also described.