Apart from the traditional internal thread-making processes such as thread tapping and thread turning, thread milling comes up with some advantages. Therefore, it has become a good choice among other alternatives in recent years. In this work, the effect of cutting conditions and thread mill diameter on the cutting temperature has been investigated in the internal thread milling of Al7075-T6 alloy. In machining processes, cutting temperature is a measure of machinability, and it is usually a barrier limiting the high-performancecutting. Therefore, cost-effective machining cannot be achieved when excessive cutting temperature occurs. Although the effect of thread mill diameter on the form error in the internal thread milling process has been known, its effect on the cutting temperature has not been investigated. A full factorial design of the experiment was used. Three levels of feed, spindle speed, and thread mill diameter were chosen for M27 thread. According to the results, there is an optimum thread mill diameter that causes the lowest cutting temperature for thepossible thread mill diameter interval. In addition, the cutting temperature increases as spindle speed and feed values increase.