Abstract: The radiation damage of biomolecules, in particular with aliphatic compound, has been extensively studied. Morpholine is a typical six-membered aliphatic heterocyclic compound. In the present work, photoionization and dissociation of the morpholine monomer and subsequent fragmentations have been investigated by synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. The vertical ionization energy of morpholine monomer is 8.37±0.05 eV, which agrees reasonably well with a theoretical value 8.41 eV of morpholine. Experimentally observed fragmentation of morpholine (m/z=87 amu) gives rise to m/z=86 amu, m/z=57 amu, and m/z=29 amu. Based on experimental and theoretical results, it is found that the m/z=86 amu is produced by losingH atom, the m/z=57 amu is formed by the elimination of CH₂O with a ring-opening process, the m/z=29 amu is generated by further dissociation of the fragment m/z=57 amu (C₃H₇N)⁺ by the elimination of C₂H₄. This finding would provide valuable insight into the photo-damage of aliphatic compounds, which may be related to living cells and other biological system.