This paper presents an experimental investigation on the RF characteristics of a 3W-class cryogenically-cooled receiver amplifier employing a gallium-nitride high electron mobility transistor (GaN HEMT) with a blue light for mobile base stations. In general, a cryogenically-cooled receiver amplifier using a GaN HEMT exhibits unstable DC characteristics similar to those found in the current collapse phenomenon because the GaN HEMT loses thermal energy at cryogenic temperatures. The fabricated cryogenically-cooled receiver amplifier achieves stable DC characteristics by injecting blue light into the GaN HEMT instead of thermal energy. Experimental results show that the amplifier achieves fine stable DC characteristics for deviation in the drain-source current from 42% to 5% and RF characteristics for a maximum power added efficiency from 58% to 68% without and with the blue light at 60 K. The fabricated amplifier is effective in reducing the power consumption at cryogenic temperatures. To the best of our knowledge, this paper is the first report regarding RF characteristics of a cryogenically-cooled receiver amplifier using a blue light for mobile base stations.