In situ Raman scattering and synchrotron X-ray diffraction have been used to investigate the effects of high pressure on the structural stability of guanidinium nitrate (C(NH(2))(3)(+).NO(3)(-), GN), a representative two-dimensional supramolecular architecture of hydrogen-bonded rosette network. This study has confirmed a structural phase transition observed by Raman scattering and X-ray diffraction at approximately 1 GPa and identified it as a space group change from C2 to P2(1). The high-pressure phase remained stable up to 22 GPa. We discussed the pressure-induced changes in N-H stretching vibration in Raman spectra and proposed that this phase transition is due to the rearrangements of the hydrogen-bonding networks.