Abstract: Fault gouge is the weakest portion of the geological materials which compose the faults in respect of mechanical properties.It is important to understand the mechanical properties of gouge for studying the dynamical problems of faults.In this paper the experimental results of mechanical properties of gouge of Yishu Fault-an important fault in North China-are reported.After measuring sixteen samples taken from five branchs of the fault,the following average mechanical quantities were obtained, the bulk modulus is 6.09×10⁴bars; the Young's modulus is 2.10×10³ bars,the viscosity is 2,68×10¹¹ Poise; the compressive strength is 35.7 bars; the shear strength is 6.12 bars and the compressive wave velocity is 1,80km/sec