Positional behavior and its relationship with substrate are crucial to understand animal morphological and behavioral adaptation, and can contribute to habitat conservation of endangered species. Colobine monkeys show diverse inter- and intra-species variations in positional behavior. Although the effects of substrate type on positional behavior are well acknowledged, this relationship has rarely been investigated in the genus Trachypithecus. We studied positional behavior of Indo-Chinese gray langurs (Trachypithecus crepusculus) in a primary subtropical evergreen broad-leaved forest for 12 continuous months, and tested the effect of substrate type on their positional behavior. The study group mainly adopted sitting during resting and feeding, and primarily used quadrupedalism (including quadrupedal walking and running) and leaping for traveling, which is different from other species of Trachypithecus living in degraded limestone habitats. Substrate type affected the langurs’ locomotor mode. The langurs tended to leap more on twigs, climb more on lianas and quadrupedal move more on wide and solid substrates. Gap-crossing and security during traveling could explain this flexible locomotion pattern observed on different substrate types. Our results provide important insights into in-situ habitat conservation and ex-situ cage enrichment of langurs.