Characteristics of rubber vibration-isolator vary evidently by its curing conditions. As the temperature and the time of vulcanization have large influences upon its characteristics, the present author prepared some test pieces, which were devided into two groups viz. (A) group, in which the temperature was kept constant and the curing time was varied, (B) group, in which the curing time was kept constant, while the temperature was varied, as the curing conditions. Then some experiments were made on the influences of these conditions. In dynamic and creep tests the following results were obtained : (1) Shore hardness increases with the advance of cure, and it decreases with the thermal degradation caused by too high temperature. (2) Real part of complex modulus and coefficients of frequency characteristics increase with hardness, and imaginary part of this modulus increases in spite of degradation. (3) Loss factor and creep deformation proved to have minimum values under same curing conditions. In conclusion the best condition for vulcanization is that of loss factor and creep are both in the minimum values.