I study the numerical and functional response by diurnal primates in Danum Valley, East Sabah, Borneo to a mast fruiting event. During the two year study period from January 2007 to December 2008, one mast fruiting occurred in June-October 2007. During the mast fruiting period, orangutans were detected twice as frequently as during the other period, in two places in Danum Valley, which are ca 15 km apart each other. On the other hand, detection frequency of other primates was stable throughout the two year period. Out of the mast fruiting period, red leaf monkeys were mostly young leaf eaters, in particular young leaves of a liana (Spatholobus macropterus, Fabaceae), but increased seed eating during the mast fruiting. Functional response is common among primates; they switch from fallback foods, such as young leaves, to fruit and/or seeds. Considerable numerical response is observed only for orangutans: they seem to migrate to fruit-rich area over several tens of kilometers. Solitariness and fat deposition of orangutans may relate to this peculiar response. Other species live in group and have stable home ranges, but solitary-living orangutans can shift their home ranges more easily than others. Orangutans are known to deposit fat, and this ability may enable them to consume large amount of fruits available during the mast fruiting events.