Abstract: There may not be many reports available in the literature about the lens and its mechanical behavior in the broken fault zone.Lens bodies are usually well developed in the sheared faults.The intensity and the deformed or broken mechanism of the broken fault zones are normally under the control of the lens bodies.Their engineering geological mechanism should be emphasized and examined in detail.This paper presents a case study on the lens and its mechanical behavior in a broken fualt zone.The findings of the study can be summarzied as follows:(1) There are various types of the lens bodies;(2) the fault lines and the mud membranes are usually distributed along the periphery of the lens body;(3)There are systems of joint cracks in the lens bodies;(4) The periphery of a lens body can be regarded as the structural surface or weak structural surface;(5) The lens body itself can be seen as a compound structure.Many mechanical behaviors of the lens body of the fault mud in the experiments of the original-location cutting and the single-axle compression are also revealed in the experiment of the original-location mechanics:(1) the periphery of the lens body has the mechanical effect of the structural surface;(2) the lens body controls the intensity and the deformed or broken mechanism.