Abstract:The nonlinear elastic hysteric behavior,which is off the Hooke¢s law and caused by the presence of structural defects in rock,such as microcracks and grain boundaries,universally exists in the measurement of stress-strain relation. The instantaneous elastic moluli of rocks in both loading stage and unloading stage vary with stain,which results in lopsided bowknot shape of instantaneous elastic modulus. Hefei sandstone,Zigong feldspar sandstone and Dali marble,saturated in 3 types of fluids with different viscosity coefficients,are tested with various frequencies by cyclic loading experiments. The angle between the loading modulus and the unloading modulus of bowknot shape is suggested to measure hysteresis of rocks,and to compare it with dissipative angle which measures hysteresis originally. Both bowknot angle and dissipative angle enhance as the area of hysteresis loop increases,and attenuation enhances as viscosity coefficient of pore fluid increases. With both the bowknot angle and dissipative angle increasing,modulus increases,and dispersion of modulus enhances as frequency increases. The variety of bowknot angle and dissipative angle with viscosity coefficient of pore fluid and cyclic loading frequency is consistent with the results of other experiments. The conclusion is drawn that the bowknot angle and dissipative angle are equivalent in measuring hysteresis of rock but are not in numerical value. The physical mechanism and significance of definition of hysteresis are discussed.