To probe how sex, age and season influence the behavior of Père David's deer (Elaphurus davidianus), we studied the diurnal activity time budget of the Père David's deer in Hubei Shishou Milu National Nature Reserve, by using a focus sampling and instantaneous recording methods. We conducted observation for six to ten days each month from Sept. 2006 to Oct. 2007. The result showed that Père David's deer have crescent activity pattern: foraged mainly at dawn and dusk and took a break at noon. They spent the most of their time resting (53.40%), followed by feeding (22.36%), moving (11.23%), alertness (5.03%), drinking (2.65%), and others (4.42%). Season and age-sex classes significantly influenced the activity time budgets. A greater proportion of time was spent resting in summer (58%) and autumn (58%). Significant difference were found in feeding, rest, alert and other activities (P<0.01) except in drinking and moving (P>0.05) among seasons. Only moving behavior showed an extremely significant difference (P<0.01) and had no significant difference in rest (P>0.05) among age-sex classes. There was an extremely significant difference between the male deer and the female deer in alert behavior (P<0.01) while the adult males spent more time on alert than the adult females. Adults and the sub-adults spent more time feeding and rest, while calves spent more time moving, and less time feeding and rest. According to the results of ANOVA analysis on the diurnal activity time budget of sex-age classes at different seasons, for all age classes, extremely significant difference were found in drinking both summer and winter, in alertness both spring and summer (P<0.01). Moreover, for adult and subadult groups, there were extremely significant difference in alertness both spring and winter, in moving both summer and winter (P<0.01).