Abstract: Background and Objective: Weather and climate conditions are environmental factors that affect the lives, comfort and health of humans either directly or indirectly. These conditions affect human behaviour in many ways and the majority of people react subconsciously to changes in climate. The study aimed to use the meteorological data to calculate and assess the trend of the thermal comfort indices for the outdoor environment in three different climates of Iran for 15 years. Materials and Methods: The necessary data required to estimate the thermal comfort indices include the average air temperature and the relative humidity per day have been acquired from the Iran Meteorological Organization for the period between the year 2000 and 2014. Three comfort indices have been utilized for an estimation which includes the humidex, the temperature-humidity (THI) and the effective temperature (ET) indices. Results: In general, from the viewpoint of thermal comfort, Arak, which represents the cold and semi-arid climate, has been experienced warm conditions in June, July and August. In the Sari, which represents the humid subtropical climate, the months of June, July and September are warm and the month of August, was very hot. In the Bandar Abbas, which represents the hot and arid climate, in May, June and September, conditions were sultry while in July and August, conditions were extremely hot. Conclusion: It seems necessary to implement precise and innovative policies and programs following climate change and global warming and the ability to prevent and reduce the risks it poses.