High outdoor-temperatures in summer months lead to the increase in indoor temperature. High indoor-temperatures with high humidity, resulting heat stress in dairy cattle. Nowadays, temperature humidity index, improved based on air temperature and relative humidity relationship, is a commonly used method for determining the effect of heat stress in dairy cattle. Under the heat stress conditions, feed intake and thus milk yield of dairy cattle is reduced. Southern Marmara is a very important region for dairycattle production. Heat stress analysis was performed for Balıkesir, Bursa, and Çanakkale, in which high milk production and large dairy cattle farms are existed. For analysis, temperature humidity index were calculated for each city. Meteorological data from 2007 to 2014 for each city were obtained from Turkish State Meteorological Service.Temperature humidity indexes for each city were calculated based on hourly average temperatures and relative humidity. On condition that no measure is taken to reduce the effects of heat stress, reductions in the milk yield of dairy cattles resulting from heat stress were evaluated separately for each city and the daily total milk losses were also analyzed.According to analysis results; daily milk losses per cow in Balıkesir, Bursa, and Çanakkale were 0.05 kg, 1.02 kg, and 0.78 kg in June; 0.42 kg, 2.48 kg, and 2.68 kg in July, and 0.30 kg, 2.70 kg, and 3.07 kg in August, respectively.Higher milk losses in June were observed in Bursa, Çanakkale, and Balıkesir, respectively. Also in July and August, higher milk losses were observed in Çanakkale, Bursa, and Balıkesir, respectively.