Ethylene plant is one of the largest plants and the thermal cracking furnace is a key unit in the ethylene plant. Since ethylene is the basic material in the chemical industry and the market situation of not only the feed and the product, but also the utility is rapidly changing, the optimal operation of the plant is important. CRACKER, a PC based software of thermal cracking furnaces is developed based on the rigorous first principles model which can reflect the effects of the operating variables. CRACKER has a user-friendly graphic interface for the convenient use and also provides a feed characterization module which can estimate the composition of conventional components from the commercially available indices. By using CRACKER, the ethylene plant operators can understand the furnace system well and easily diagnose the current operation and thus can make both the optimal operation strategy according to the market and the optimal co-cracking strategy according to the characteristics of the purchased naphtha. Also in the purchase of naphtha, the feed characterization module can help to determine the better naphtha for the plant based on the price of the naphtha and products.