TibetangazelleProcapra picticaudata is a threatened and endemic species to the Qinghai-TibetPlateau. With the method of group scan sampling, we observed the behaviours of males and females of the gazelle in the two summers of 2005 and 2006, in order to test the groupsizeeffect on group vigilance. We found that male gazelles were significantly more vigilant than the females at both group scan level (percentage of individuals scanning during a session) and group scan frequency (percentage of intervals with at least one individual scanning). We also found a negative correlation between group scan level and groupsize and a positive correlation between group scan frequency and groupsize, showing the groupsizeeffect on vigilance was testified in Tibetangazelle. The predation factor might be the main driving force for the groupsizeeffect.