The influence of various calcium carbonate on the physical properties of natural rubber have been carried out extensive experiments on compounding. Final evaluation was made throughvarious kinds of physical. test-tensile strength, modulus, tear strength and hardnees-including particle size. The results are as follows: 1. Generally, the tensile strength, modulus and tear strength showed the similar tendency in various calcium carbonate compounding. 2. The maximum volume of general using have been obtained with 150 phr calcium carbonate in NR-CaCO₃ compounding. 3. The particle size of domestic calcium carbonate showed 1.0μ-2.6μ.