In this study are used heat index, effective temperature, wet bulb globe temperature, and apparent temperature indices. Air vapor pressure, temperature, relative humidity, and wind speed data are very effective meteorological parameters over the sensitive temperature. Daily mean temperature (A degrees C), relative humidity (%), wind speed (m/s), and vapor pressure (hPa) data are used in this study. Using the thermal indices is calculated to long-term data from 49 stations distributed over the western part of Turkey. The Mann-Kendall test is applied to identify statistically significant trends at each series of thermal indices. The results of this study have shown that there are marked spatiotemporal variations in the thermal indices in the western part of Turkey. Increasing trends are stronger in summer compare with other season. Analysis are displayed that the increasing trends have shown warm-hot and discomfort days, especially during the summer season. High values of these indices are mainly connected to Azor high-pressure system and Monsoonal circulation and weakly etesian winds in summer.