We have measured the high-resolution vibrational spectra of a thietane (trimethylene sulfide) cation in the gas phase by employing the vacuum ultraviolet mass analyzed threshold ionization (VUV-MATI) spectroscopic technique. Peaks in the low frequency region of the observed MATI spectrum of thietane originate from a progression of the ring-puckering vibrational mode (typical in small heterocyclic molecules), which is successfully reproduced by quantum-chemical calculations with 1D symmetric double-well potentials along the ring puckering coordinates on both the S-0 and D-0 states, the ground electronic states of neutral and cation of thietane, respectively. The values of the interconversion barrier and the ring-puckering angle on the S-0 state, the parameters used for the quantum-chemical calculations, were assumed to be 274 cm(-1) and 26 degrees. The barrier and the angle on the D-0 state, however, are found to be 48.0 cm(-1) and 18.2 degrees, respectively, where such small barrier height and puckering angle for the cation suggest that the conformation of thietane cation on the D-0 state should be more planar-than that of the thietane neutral.