Many studies have shown that with the support of an upright backrest, the response of seated human body to the Whole-body Vibration is nonlinear subject to the change of the vibration magnitude. However, although many seats in modern vehicles have inclined backrest, there has been very little study on whether the inclination of backrest could further affect the nonlinearity of human responses to vibration caused by the increase of excitation magnitude. In this study, the in-line (vertical) and cross-axis (fore-aft) apparent masses at the seat pan and backrest were studied with twelve subjects supported by a rigid backrest with three inclination angles (upright, 10° backwards and 20° backwards) adjusted in sequence while exposing to vertical vibration at different magnitudes (0.25, 0.5 and 1.0 ms -2 r.m.s.). The results show that the nonlinear behaviour of apparent mass observed when the backrest was upright also existed when the backrest was inclined. It was further found that the effect of the magnitude of excitation on the apparent mass measured at the backrest was influenced by the inclination of the backrest.