The investigations have been carried out to study the characteristics of limestone using in Korean cement industry with prime interests in the dissociation and thermal phase changes of limestone between 90$0^{circ}C$ and 140$0^{circ}C$ The range of decomposition temperature of limestone was 840-87$0^{circ}C$ and it was considered that the temperature was lowered by impurities in the limestone. The more the impurities and the finer the grain size of quartz in limestone the lower temperature was applied in forming $C_2S$ mineral the temperature of about 100$0^{circ}C$. The major clinker minerals such as $C_3S$, $C_2S$, $C_3A$ and $C_4AF$ were formed in most of limestone when the firing temperature was up to 130$0^{circ}C$