This part of the article deals with the marine and non-marine Mesozoic stratigraphy, the Mesozoic folded mountains, the Cenozoic history of Thailand and West Malaysia, the summary of the geological history of the region and my retrospect of its research. The marine Triassic formations are distributed on the two sides of the Chiangmai axis in Thailand and of the Malayan main range, the eastern ones of which contain volcanic rocks. In the Malayan peninsula the Triassic and older formations disturbed by orogeny are overlained by the non-marine Tembeling formation. In Thailand the Lampang group on the eastern side was folded in the late Triassic age, while the continuous Triassic and Jurassic formations constitute the Mae Moe group in the Burmese-Thailand border. In East Thailand the Khorat group in the hinter basin of the Pak Lay folded belt consists of upper Triassic-Jurassic paralic sediments and Cretaceous limnic ones. While they are sub-horizontal, the Jurassic and older strata on the Burmese frontier are deformed by the late Mesozoic orogeny and overlain by the Cretaceous red beds. The Tembeling basin in the peninsula was also deformed by this orogeny. At the same time its east side warped down forming the Gagau basin. The older and younger Mesozoic orogenies were associated with granitic intrusions. The Mesozoic folded mountains thus well consolidated were destructed repeatedly by the Cenozoic block movements till at length the present topography was yielded.