Abstract: A series of dynamic deformation experiments have been carried out on both dry and wet marble samples, by means of the torsional Kolsky bar modified for simple shear, to explore the influence of pore water on the strength or fracture stress at several high strain-rates (373s-1 to 1736s-1). The temperature was 20℃, and confining pressure was 0.1 MPa. An alloy (AZ8GU) was used as the material for torsional bars (G = 26800 MPa, = 2800 kg/m3). The incident, reflected and transmitted torsional pulses were recorded using an oscilloscope. The shear stress, strain and strain-rate were evaluated from the recorded torsional pulse signals by using a mini-processor.Both dry and wet marble samples show brittle deformation behavior. The relation between strain-rate and estimated experimental fracture stress is: = 26.531n-150.16 for the dry sample and is:r = 27.211nr-143.76for the wet one, where is in MPa.According to the data of the present experiment and previous ones (Rutter and Mainprice, 1978; Henry, 1978), the possible deformation rate controlling processes at low temperature are discussed, and it is suggested that in the wet experiments at low strain-rate (e.g. 10-9 s-1) the effect of water on rock deformation is controlled by a pressure solution process, while deformation at intermediate strain-rate (10-7 to 10-3s-1) it is controlled by the kinetics of water assisted stress corrosion. During deformation at high strain-rate (>10-3s-1) the crack velocity is too high to give enough time for chemical reactions between water and minerals at the crack tip. But the noteable difference in mechanical behavior between dry and wet rocks may be interpreted as an effective shear stress effect.The geological implications of the results are discussed, and particular emphasis is given to their significance for the study of earthquakes.