Abstract: This paper presents an experimental study on the vertical transmissibility of a passive air-suspension seat under tri-axis vibration. The vibration magnitudes were 0.25 m/s² r.m.s., 0.5 m/s² r.m.s., or 1.0 m/s² r.m.s. in the z-axis and 0 m/s² r.m.s., 0.5 m/s² r.m.s., or 1.0 m/s² r.m.s. in both the x- and y-axis, and backrest inclination angle was 0°, 10°, and 20°. The results show that both the horizontal and vertical vibration magnitude led to a decrease in the resonance frequency of the seat transmissibility, which affected the value of Seat Effective Amplitude Transmissibility (SEAT). The resonance frequency of the seat transmissibility was also found to be different with changing backrest inclination angles when the vertical vibration magnitude was 0.25 m/s² r.m.s. Furthermore, interaction existed between the vertical and horizontal vibration affecting the seat transmissibility. When the vertical vibration magnitude was at a higher level, the degree of the change in the resonance frequency due to the increase in the horizontal vibration magnitude from 0.25 m/s² r.m.s. to 1.0 m/s² r.m.s. became smaller, and vice versa.