The effect of high pressure on the structural stability of oxamide has been investigated in a diamond anvil cell by Raman spectroscopy up to similar to 14.6 GPa and by angle-dispersive X-ray diffraction (ADXRD) up to similar to 17.5 GPa. The discontinuity in Raman shifts around 9.6 GPa indicates a pressure-induced structural phase transition. This phase transition is confirmed by the change of ADXRD spectra with the symmetry transformation from P (1) over bar to P1. On total release of pressure, the diffraction pattern returns to its initial state, implying this transition is reversible. We discuss the pressure-induced variations in N-H stretching vibrations and the amide modes in Raman spectra and propose that this phase transition is attributed to the distortions of the hydrogen-bonded networks.