a. Department of Geosystem Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan

It is well known that uniaxial compressive strength of rock increases with loading rate. Loading rate depen-dency of Young's modulus has been also studied by many authors, however, only a small part are clarified.
In this study, testing method are slightly modified to reduce scatter of data, in the testing, Young's modulus at 50% of the peak strength is normalized by that at 10% of the peak strength which is preliminarily obtained undera predetermined loading rate. It was found that this testing method is effective and promising.
Seven Japanese rocks were examined. It was found that an andesite, a sandstone and two tuffs exhibited loading rate dependency of Young's modulus. For example, in case of Sanjome andesite, Young's modulus increased 2% at every ten fold increase of loading rate. On the other hand, Young's moduli of a marble, a granite and a sandstone very slightly decreased with increase of loading rate.
With aid of constitutive equations, time dependency of modulus was discussed. It was found that irrecoverable strain delayed to extend when loading rate was high. That is possibly the main reason of increase of Young's modulus with increase of loading rate.