An experimental investigation has been carried out to study the effect of addition and substitution of micro calcite on strength and durability characteristics of High Performance Concrete (HPC). Various tests were conducted on HPC specimens including compression on cubes, split tension on cylinders and flexure on prisms to evaluate mechanical strength, water permeability, sulphate resistance and chloride resistance on cubes to evaluate durability characteristics. A total of 396 concrete specimens of standard sizes were cast with and without micro calcite and tested at different ages (i.e. 7, 14 and 28 days) to evaluate strength and durability characteristics of concrete following relevant Indian standard code procedure. Two different cases of incorporation of micro calcite in concrete viz. addition case and substitution case were considered, and in each case, five different dosage levels of micro calcite inclusion in concrete viz. 5%, 10%, 15%, 20% and 25% by weight of cement were considered. Test results indicate that incorporation of micro calcite in concrete either by addition or by substitution up to a certain level improves strength, durability and strength-to-weight ratio of concrete markedly, and any further inclusion beyond that level does not enhance the above properties of concrete. In addition, the improvement in mechanical strength at early age is higher than that at later ages for both cases of addition and substitution. Furthermore, optimum dosage level of micro calcite to achieve maximum gain in strength, durability and other engineering properties of concrete are 10% and 15% for addition and substitution cases, respectively.