Regarding climatic diversity in Iran and the dependence of many outdoor activities (occupational, recreational, tourism attraction, etc.) on climatic conditions, this study conducted aiming to evaluate thermal comfort fluctuations in Iran. Thermal comfort fluctuations in different climates of Iran were investigated between 1985 and 2014 using recorded meteorological data in 40 synoptic stations. The Mann-Kendall test and Sen’s Slope were used to analyse the changing trends of the Humidex and to determine the actual slope of a trend over time, respectively. In overall, the southern areas of Iran have the longest period with unfavourable and extremely unfavourable climatic conditions in this study. Conversely, in the northern half of Iran, especially in North West areas, the appropriate summer season provides the best climatic conditions for outdoor activities.