The plasma temperature of Ar gas in hollow cathode discharge were measured. This is done by measuring the line profile of the 1s₈-2p₈ transition in Ar, using a single-frequency diode laser. Low power diode lasers have been successfully used for investigation of the line profiles of Ar transitions in hollow cathode discharges. It turns out that the plasma temperature of Ar is 640∼783 K in the discharge current range at 7∼10 mA.