In order to clarify the strain rate effect on mechanical characteristics of concrete, rapid triaxial high compression tests and rapid hydrostatic loading tests are conducted for cylindrical concrete specimens in the same manner as the static tests in Part 1 except forloading rates. In the rapid loading tests, the loading rates are applied corresponded to average uniaxial strain rates (D1) epsilon/sub 1/ = -1.44 x 10/sup -3//sec, (D2) epsilon/sub 1/ = -2.40 x 10/sup -2//sec and (D3) epsilon/sub 1/ = -2.54 x 10/sup -1//sec. The effect of strain rate magnitude on the intial elastic moduli and failure strength are discussed, then the rates of increase of them due to octahedral strain rates are expressed by empirical equations. The tangent elastic moduli, which are described below, are derived by introducing this effect to the static tangent moduli presented in Part 1. K/sup t/ = K0center dotk(epsilon/sub oct/)center dot..cap alpha../sub V/(epsilon/sub oct/)center dot..cap alpha../sub D/(phi/sub d/) G/sup t/ = G0center dotg(epsilon/sub oct/, ..gamma../sub oct/)center dot..beta../sub V/(epsilon/sub oct/)center dot..beta../sub D/(phi/sub d/) in which, K0 = initial bulk modulus, G0 = initial shear modulus, epsilon/sub oct/ = octahedral normal strain, epsilon/sub oct/ = octahedral normal strain rate, ..gamma../sub oct/ = octahedral shear strain rate, k(epsilon/sub oct/) and g(epsilon/sub oct/, ..gamma../sub oct/) are the increase coefficients due to strain rates. phi/sub d/ is corresponded to phi defined by expanding failure curve due to strain rates. Then, ..cap alpha../sub V/(epsilon/sub oct/), ..beta../sub V/(epsilon/sub oct/), ..cap alpha../sub D/(phi), ..beta../sub D/(phi) and phi are given in Part 1.