Two-dimensional (2D) plasmons in grating-gated AlGaN/GaN heterostructures were investigated by terahertz time domain spectroscopy observing the distinctive minima and inflection points in the transmission amplitude and phase spectra, respectively. The main features of 2D plasmons at temperature of 80 K were the fundamental mode position at 2.2 THz (grating period of 600 nm, filling 50 %), the quality factor up to 8 and the modulation of transmission amplitude and phase up to 50 % and 18 deg, respectively (period 1000 nm, filling 80 %).