The magnetotransport measurement is performed on a GaN/AlxGa1-xN heterostructure sample in a low temperature range of 1.4-25 K and at magnetic fields ranging from 0 T up to 13 T. Magnetoresistance of a two-dimensional electron gas confined in the heterostructure is investigated. The negative magnetoresistivity in the whole magnetic field range originates from the electron-electron interactions (EEIs), while the positive magnetoresistivity in the high field range results from the parallel conductance. The EEI correction terms, as well as the concentration and mobility of the parallel channel are obtained by fitting the experimental data. Furthermore, another method of calculation is used to check their accuracy.