We conducted an experiment in the Beijing Milu Park to study the social behavior of male Père David's deer, and related social behavior to social position and serum testosterone level of the stags during rut. We classified the stags into three rank classes according to their rutting behavior: ‘harem master’, ‘challenger’ and ‘bachelor’. We monitored the behaviors of four ‘harem masters’, five ‘challengers’ and five ‘bachelors’, and analyzed serum testosterone levels in blood samples of those 14 stags using radioimmunoassay. We defined the effectiveness value, E = A/T, to assess the effectiveness of herding or mating attempts made by stags (‘T’ represents the frequency of herding or mating attempts made by a stag and ‘A’ represents the frequency of herding or mating attempts accepted by hinds). We found that: (1) the ‘harem masters’ and the ‘challengers’ displayed more frequent rut and locomotive behaviors but fewer ingestion behaviors than the ‘bachelors’; (2) serum testosterone levels in the ‘harem masters’ and the ‘challengers’ were higher than that in the ‘bachelors’; (3) effectiveness value of herding attempts differed significantly between the three types of stags, being highest in the ‘harem masters’ and the lowest in the ‘bachelors’; and (4) effectiveness value of mating attempts was significantly greater for the ‘harem masters’ than for the ‘challengers’. We conclude that: (1) reproductive behavior of the Père David's deer stags is strongly associated with social rank; (2) social roles of Père David's deer stags during the rut are related to the testosterone secretion; and (3) rank class affects the mating opportunity of the stags.