A fundamental study on vibration characteristics of the air cushion craft was performed taking dynamic behavior of the pneumatic seal structure into consideration. A linearized mathematical model of the air cushion craft dynamics was derived. The effect of each parameter on the dynamic stability was obtained analytically and experimentally from the view point of negative resistance excitation system. The conclusions are as follows : 1) The dynamic stability of pneumatic seal structural vibration depends on the air transmission line characteristics and the seal configuration. The effects of these parameters on the stability criteria of the system are numerically examined. 2) In the model test, the self excited vibration caused by the instability of the pneumatic seal system was observed in some range of the inlet area of fan and feedhole area in the air chamber. 3) The calculated fundamental natural frequency for air cushion craft model agreed very well with model test results. The stable region varied in accordance with feedhole area in the model test, and a similar tendencies were also observed in the numerical analysis. A further study is necessary to discuss the difference between model test results and numerical analysis results.