Within 30 Hz, the discomfort caused by whole-body vibration in rotational direction is higher than vertical vibration at similar equivalent magnitude. Roll vibration, in particular, produces greater discomfort comparing with pitch and yaw vibrations. It is critical to understand the biodynamic characteristics of seated human body under roll vibration for both comfort assessment and vibration control. Experiments are carried out to obtain the biodynamic response of seated human body under random roll vibrations at four r.m.s. magnitude levels. It is found that the principal resonance in the roll apparent inertia is about 1 Hz, but varied from 0.7 to 1.5 Hz depending on the magnitude of vibration (0.5 to 2.0 rad/s2), and the secondary resonance locates around 3 Hz with a much lower modulus. It is noted that the human response to roll vibration has some features in common with that in the lateral direction. Two lumped parameter models are developed and calibrated to study the correlation between the two excitation axials. The equivalent relationships of magnitude and phase between roll and lateral vibrations are obtained on condition that they produce similar rotational responses of the upper human body. It suggests an equivalence approach between translational and rotational vibrations that can benefit the comfort assessment when exposed to multiaxial excitations.