Global warming is a challenging issue among the world's climatologists, embraced even by politicians and the common people. In this study, the trend of temperature changes in 31 metropolises of Iran, which are generally the provincial centres of the country, has been studied in a statistical period of 35 years (1985-2019). We found that there are more temperature changes in all metropolises of Iran in winter than other seasons. In this season, the maximum temperature parameter shows a significant increase, as evidenced in Ardabil with about 9 degrees Celsius. Also, the highest amount of temperature increase in all seasons has occurred in the western and northwestern part of Iran, being of mildly cold to extremely cold climate in Iran. In general, the temperature changes more intensively from the south to the north regions. It is interesting that in the southern and coastal cities of Iran, such as Ilam, Bushehr and Shiraz, in some months, the temperature keeps decreasing. The increase in temperature of metropolises has been much more regular for the maximum temperature than the minimum temperature. Strangely, unlike all the months in November, the maximum temperature has decreased in most cities in Iran.