Abstract: The aim of this study was carried out to determine the critical periods of year for dairy cows using the temperature humidity index. The 30 dairy cows were housed in tie-stall barns in Kahramanmaras, Turkey. The dairy cows slightly experienced the heat stress during time from mid May and the end of October. Farmers should take measurements when the THI is above 72 during the summer months to prevent the losses in milk production and changes in milk composition, milk somatic cell counts and mastitis frequencies.