The environmental change of Yong-nup in Mt. Dae-Am, which is located at the northern part of Kangwon-Do, Korea, was assesed with peat sedimentary carbon and nitrogen isotope analysis. The surface layer of the peat (0 ${sim}$ 5 cm) was 190 year BP, and the middle layers (30 ${sim}$ 35 cm and 50 ${sim}$ 55 cm) were 870 year BP and 1900 year BP, respectively. Bulk sedimentation rate was estimated to be about 0.4 mm $year^{-1}$ for 0 cm to 30 cm and 0.15 mm $year^{-1}$ for 35 cm to 50 cm. The $^{14}C$ age of the bottom sediment (75 ${sim}$ 80 cm) collected and measured in this study was about 1900 year BP, although it was measured that the $^{14}C$ of the lowest bottom sediment in Yong-nup was 4105 ${pm}$ 175 year BP (GX-23200). Since the $^{14}C$ ages for 50 ${sim}$ 55 cm and 75 ${sim}$ 80 cm layers were almost the same as 1890 ${pm}$ 80 fear BP (NUTA 5364) and 1850 ${pm}$ 90 year BP (NUTA 5462), respectively, we have estimated that the deep layers (55 ${sim}$ 80 cm) in the high moor were the original forest soil. The low organic C and N contents in the deeper layers supported the inference. The sediment of 50 ${sim}$ 55 cm layer contains much sandy material and showed very low organic content, suggesting the erosion (flooding) from the surrounding area. In this context, the Yong-nup, high moor, of Mt. Dae-Am, might have developed to the sampling site at about 1900 year BP. The ${delta}^{13}C$ values of organic carbon and the ${delta}^{15}N$ values of total nitrogen in the peat sediments fluctuated with the depths. The profile of ${delta}^{13}C$ may indicate that the Yong-nup of Mt. Dae-Am have experienced the dry-wet and cool-warm period cycles during the development of the high moor. The ${delta}^{15}N$ may indicate that the nitrogen cycling in the Yong-nup have changed from the closed (regeneration depending) system to the open (rain $NO_3;^-$ and $N_2$ fixation depending) system during the development of the high moor.