Jeju Island is a volcanic island which has begun to form by the volcanic activities from pliocene to quaternary in geologic age. The main factors of landform in this island are composed of more 79 times of the lava eruption, patterns of volcanic activities by pyroclastic eruption, the diffential erosion being caused by the dissimilarity of rocks and the fluvial and the marine erosion carrying with the formation period and the upheaval. The original landform of this island is an Aspite whose top is Mt. Halla, but near the top of Mt. Halla is like a Tholoide. This island have about 400 parasitic cones. The river system of this island has a radial pattern according the shield-shaped landform whose top is Ht. Halla. Development of river system in the eastern and the western slopes in this island is poor, because these, in comparison with the northern and the southern ones, are relatively long and smooth and have the broad lava plateaus Many rivers, therefore, run down northerly or southerly. The tip of headward erosion toward the pick of Mt. Halla reaches 1600tn above the sea-level, and mosr. of the rivers have the characteristics of the youth stage by the very activing downward erosion which reaches near the coastline. The northern and the southern slopes in this island show the different factors in precipitation, geological distributions, and amounts of upheaval. According to the geological observation, cornparision and analysis of the two slopes this island, it comes to the conclusions as follows: 1. The precipitation, the formation periods of lava, the difference in amounts of upheaval, the diffential erosion being caused by the dissimilarity of rocks, are the main factors which bring the different landforms in the northern and the southern slopes of this island. 2. The southern slope operated more erosion than the northern under the influence of different precipitations. The gap between two slopes in precipitations is ,346. 3mm a year on theaverage in these 16 Years. 3. The southern slope have also the larger relief energy with a large amount erosion, because it is covered more broadly, as compared with the northern slope, with the sedimentary rocks that are erupted at a relatively former period and the trachyte and trachy-andesites that are weak relatively against erosion. 4. Because of the different relief energy, the southern slope is operated more erosion than northern and becauset the ratio of upheaval is rapidy than ratio of erosion, the deeping is operated actively near to coastline. In the northern slope, we can find the northern slope, we can find the trace of marine erosion as far as 70m, but, in the southern slope, even 180m above the sea level. 5. Regarding the coastal dune, the northern slope is more developed than the southern by reason of the direction of the monsoon. In this island, the northwesterly monsoon in winter is stronger than the southeasterly one in the summer season. 6. The differences in development of rivers on the two slopes are as follows: 1) About the curvature, the amounts of erosion, the drainage networks of rivers, the southern slope is larger and more developed than the northern. 2) About the degree of upheaval, the southern coast is more remarkable than the northern. 3) This is caused by the blocking that arises the trachytes or the trachy-andesites in which the columnar joint are will developed.