The Kii Mountains belongs to the outer zone of the south of the medial line that divides the western Japan into the north part and the south part, and it is formed of mainly the mountain ranges which run from north to south including the Omine Mountain Range that rises at the middle of the Kii Mountains, the Obako Mountain Range that rises at the west of the Omine Mountain Range and at the opposite side of Totsu-kawa River which separates from Omine, and the Daiko Mountain Range that rises at the east side of Kitayama-gawa River which separates from Omine as well.
紀伊山地は西日本を南北に二分する中央構造線以南の外帯に属し、南北方向に走る山脈、すなわち、紀伊山地中央にそびえる大峯山脈、十津川を挟んで大峯山脈の西側の伯母子山脈、北山川を挟んで東側の台高山脈を軸とする。