Critical environmental elements for long term preservation of a paper are temperature, humidity, dust, light, contaminants in air, and mold. Especially, temperature and humidity need special care, because they can not only directly degrade paper but also affect the degradation of the other elements. Therefore methods to monitor variation of temperature and relative humidity were studied. One of the methods was to use preservation index for evaluating preservation environment. Since the evaluation for preservation environment of a stack room investigated firstly by D. K. Sebera, PI(preservation index) and TWPI(Time-Weighted Preservation Index) by IPI is made to evaluate relatively preservation environment in a stack room. T. Padfield developed method which could calculate easy PI and TWPI. In this study, the preservation environment of stack rooms for paper storage in NAK(National Archives Korea) was evaluated by PI and TWPI. PI and TWPI of stack rooms for paper storage in NAK maintained good condition but PI depended on season. Then the preservation environment of stack rooms for paper storage in NAK was required to maintain continuously PI and TWPI regardless of season change.