Abstract: Abstract: Northeastern China is one of the dominating areas of dairy production. In 2016, a total of 2.375 million dairy cows were kept within Heilongjiang, Liaoning and Jilin Provinces, annually producing over 7.42 million tons fresh milk. Northeastern China is characterized by an extremely cold climate in winter, where the dairy barn design and management are typically focusing on the structure insulation, wind and freezing prevention in winter in particular. Big data also show that dairy production in this area is often threatened by heat stress in summer, while little attention has been paid on the assessment of heat stress and its impact on dairy cow health and its production performance. In this paper, field measurement was conducted in a typical dairy farm with naturally ventilated barns and open playing field, which the cows had free access to, in Heilongjiang Province for the whole summer of 2017. Dry-bulb temperature, relative humidity, wind velocity and black-bulb temperature were continuously monitored and recorded using the data loggers well distributed inside and outside the barn, which housed 820 milking Holstein cows, 273 m long and 36 m wide, and was oriented from north to south. The intra-vaginal data logger, which integrated a body temperature data logger with a blank controlled internal drug-releasing (CIDR) device, was applied to measure the core body temperature (CBT) of the cows. Behavior information, including the lying, standing and walking frequency and duration, was collected by the pedometers. The device was self-developed in the lab and attached to the foreleg of cows by bandages. Milk production information was directly gained by GEA milking system. Based on the continuous measurement on thermal environment indices, vaginal temperature of the cows, behavioral information and milk production, assessment of heat stress of dairy cattle and its effect on animal behavior and milk yield was carried out. Temperature-humidity index (THI) and black globe humidity index (BGHI) were chosen to evaluate heat stress for the cows inside and outside the barn. The results showed that the cows experienced heat stress that was from mild to moderate in the measurement evaluated by indoor and outdoor thermal environment indices, which accounted for 40.9% and 17.9% of the total time spent by the cows inside the barn, and 20.5% and 29.8% on the open playing field, respectively. The cows on the open playing field were suffered from heavier heat stress because of the solar radiation. Moderate heat stress usually occurred during 12:00-20:00, and thus a cooling measure between 16:00 and 20:00 is still suggested for the cows although the ambient temperature was decreasing during this period. When heat stress changed from mild to moderate, CBT of the cows increased from 38.8 to 39.3 ℃, and a time lag between CBT and thermal environment temperature was observed in this paper. Meanwhile, lying time proportion of the cows statistically reduced from 51.3% to 42.3% when THI kept increasing from no heat stress to moderate, and the standing time proportion increased accordingly. Compared to the daily average milk yield of cows when THI was lower than 75, the production fell by 9.2%, and when THI reached above 75 (P<0.05), it was decreased to 25 kg for each cow on the average. Free access to the open playing field during nighttime for cows can alleviate heat stress of the cows, and hence reduce the milk production loss.