In a process for calibrating the temperature control unit of a vertical gradient freeze crystal growth oven, instead of the fused material a test body (3) is used in the oven (1) that does not melt at the oven temperature, that has a heat conductivity comparable to the fused material and a central bore (4). After turning on the resistance heaters (8, 9, 10) of the oven (1) the temperature at the level of the individual control temperature indicators (11, 12, 13) of the oven (1) is measured via a reference temperature indicator (5) than can be fully inserted into the bore (4) and subsequently the output of the respective resistance heaters (8, 9, 10) is set to a desired temperature value. In the process the difference to the temperature of the respective control temperature indicators (11, 12, 13) is determined. After removing the test body (3) and the reference temperature indicator (5) the oven (1) is operated according to the control temperature indicators (11, 12 13) taking into consideration the previously determined differences between the reference temperatures and the temperature values of the control temperature indicators (11, 12, 13).