In this study, white yak’s milk samples from three pastoral areas in Tianzhu county, Gansu province, collected at four different growth stages (withering, the final stage of withering, reviving and green) of forage grasses as well as from female yaks of different parity were analyzed by GC–MS for fatty acid composition. The results showed no significant difference in fatty acid content of yak’s milk among three pastoral areas or among 5 parities (P ＞ 0.05). The SFA:USFA ratio of yak’s milk at the withering stage, the final part of withering, the reviving stage and the green stage was 1.86, 1.75, 1.62 and 1.32, respectively. Lower amounts of SFAs but higher amounts of USFAs were found in white yak’s milk as the temperature increased and forage became more mature.