The purpose of this study is to revise the HDD(Heating Degree-Days) of main cities of Korea because the outside temperature rise have been accelerated by global warming recently. Now our HDD(Heating Degree-Days) for the utility design of district heating system had been `established in 20 years ago. Therefore new heating degree days for main cities of korea had been required and determined using long-term measured outside weather temperature data during 30 years. For the analysis of HDD, five different base temperatures ranging from 24 to $16^{circ}C$ were chosen in the calculation of heating degree days. And new yearly heating degree days of 15 cities of korea were given in the tabular form.