The I-V characteristics of AlGaN/GaN high electron mobility transistors in the temperature range between 100 K and 300 K are studied. It is found that both the maximum drain-source current and transconductance decrease with the increase of temperature. Decrease of the electron mobility with increasing temperature is considered to be the main cause for that condition. The threshold voltage shows a forward shift, which can be explained by the increase of Schottky barrier with increasing temperature. It is found that at VGS = 0 V the drain-source current reduces with the ascending temperature, which should be due to the variation of the electron mobility with the temperature. While at VGS = −5 V the drain-source current is found to increase with the ascending temperature, it is suggested to be caused by the positive temperature coefficient of the electron transport in the depleted region.