[Objectives]  To investigate whether the stag beetle Dorcus hopei -coexists male trimorphism due to their remarkable variation in mandible and elytra length, and if so, to determine the relative success of each morph in mate competition. [Methods]  90 male specimens were classified into three putative morphs on the basis of mandible and elytra length. Male mating behavior was then filmed using an infrared camera and the relative competitive advantage of each morph assessed by measuring how long each took to approach female. [Results]  -type males have well-developed mandibles (14.70-18.84 cm long) and elytra (22.14-25.86 cm long), whereas -type males have rudimentary mandibles (4.88-8.99 cm long) and elytra (15.42-19.40 cm long), and -type males are intermediate in mandible (11.67-15.99 cm) and elytra (19.24-23.77 cm) length.  males spent more time in close proximity to females than  and  males, and  males spent more time in close proximity to females than  males. When  males coexisted with other two male morphs, they always took more time to approach females. The observations implied the males at a disadvantage might benefit indirectly from the strong competition between two advantaged males.. [Conclusion]  Our results suggested that the males of D. hopei are trimorphic, with three distinct male morphs simultaneously present in the population. Morphs could be ranked in terms of competitive superiority as follows: >>. We speculated that males as the less strong type would gain opportunities by drawing on the competition between two competitive morphs.. Further studies will be needed to understand the mating behavior and tactics among three male morphs. This is the first report of male trimorphism in the Chinese stag beetle. Our findings provide insights into the understanding and knowledge of multiple phenotypes coexisting within the same species.