Paleobiogeography of the Late Paleozoic ammonoids in Southeast Asia is discussed on the basis of the fossil records from Thailand and Malaysia. The index of faunal resemblance is expressed as 100 C/N1, where C is the number of taxa common to both, and N1 is the total number of taxa (at a specified level) in the first, smaller (or equal) of the two faunas or samples compared. The ammonoid faunal resemblance between Sibumasu/Indochina and other regions are studied. The fluctuation of the curves between Sibumasu and several regions supports the previous opinions that the Sibumasu terrane separated from the margin of Gondwanaland in Early to Middle Permian. However, the faunal resemblance between Indochina and South China does not show close relationship from Pennsylvanian to Middle Permian. This result is opposed to the previous opinion.