High air temperatures at the grain-filling stage are a critical constraining factor on winter wheat yields in Northern China. As global warming progresses, the more frequent occurrence of high temperatures may lead to large fluctuations in winter wheat yield in this region, which may be a threat to the food security of China. To evaluate the effects of unusually high temperatures at the mid to late grain-filling stage on winter wheat yield in Northern China, open field experiments with a short-term high temperature treatment were conducted at the Gucheng Ecometeorological Observation Experimental Station of the Chinese Academy of Meteorological Sciences (Dingxing County, Hebei Province, 39°08'N, 115°40'E) in 2012 and 2013. High air temperatures under the winter wheat canopy were induced using infrared radiator heaters. Two temperature increase treatments were applied each day from 11:00-15:00 for 6 or 9 days, during the mid to late grain-filling stage of winter wheat. In each temperature increase treatment, three levels of temperature increase above the ambient temperature were applied. Canopy air temperatures were recorded and the weights of 1000 kernels were sampled. High temperature stress at the mid to late grain-filling stage of winter wheat reduced the grain-filling rate and shortened the grain-filling period, resulting in a significant reduction in grain yield. After accounting for covarying factors, a significant negative linear relationship was found between grain weight and midday canopy air temperature: when the average midday canopy air temperature over 9 days rose to 32.9℃ and 35.9℃, or the average midday canopy air temperature over 6 days rose to 33.8℃ and 37.6℃, winter wheat grain yields decreased by 10% and 20%, respectively, compared to grain yields at the average midday canopy air temperature during the grain filling stage over the last 20 years in the region (30℃). A steeper drop in grain weight owing to high temperature stress was not observed because of the limited temperature increase. This study suggests that the magnitude of high temperature stress effects on winter wheat grain yields depends on both the strength and duration of the stress, which can be represented as the effective accumulated heat (summation of the hourly temperature differences above a threshold value during the mid to late grain-filling stage of winter wheat). There were significant negative linear relationships between the normalized grain weight of winter wheat and the effective accumulated heat above 30℃ during the mid to late grain-filling stage (P ≤ 0.05). This shows that the high temperature stress index is a useful parameter for quantitative evaluation of the impacts of high temperatures at the grain-filling stage on winter wheat yield. To cope with such high temperature damage at the mid to late grain filling stage, it will be necessary to adopt heat tolerant varieties and to develop cultivation and management measures to mitigate the effects of heat damage.