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The yak is the primary herbivore grazing on the Qinghai-Tibetan plateau, an area characterized by extremely harsh conditions and a very short vegetation growing season. Herbage quantity and quality peak in summer when it is lush and green, but is dry and withered in winter when it is scarce and of poor quality. Traditionally, yaks graze only natural pasture all year without supplements and have adapted well to the extreme winter conditions. Grazing behaviour has been used as an indicator for grazing management decisions and, in this study, was measured in five 4 to 8 years old yak cows suckling calves during summer and winter on the Qinghai-Tibetan plateau. The diurnal behavioural patterns were similar in the two seasons. Grazing time and jaw movements per hour increased at dawn (07:00), remained relatively steady and then decreased at dusk (19:00), whereas ruminations, mastications and boluses per hour exhibited an opposite trend and decreased at dawn, and then increased after dusk. Seasonally, with a decrease in herbage quality from summer to winter, yaks decreased grazing and ruminating times and walking distance (P < 0.05) and increased drinking and idling times (P < 0.05). Air temperature was the primary meteorological variable affecting yak behaviour and was correlated positively with grazing time, ruminating time, the rate of mastications while ruminating, boluses formed and mastications per bolus (P < 0.05). In addition, wind velocity and precipitation significantly influenced the grazing behaviour of yaks (P < 0.05). We concluded that both meteorological and forage conditions affected the grazing behaviour of yaks and that the general reduction in grazing activities in winter could have been a strategy to reduce exposure to the harsh conditions and, consequently, reduce energy expenditure for foraging and for thermoregulation.