Abstract: This paper presents a study on the biodynamic characteristics of seated human body under roll-vertical vibration. Initially, the dynamic responses of eight subjects under single-axis random roll vibration was measured. The apparent inertia was introduced, defined as the frequency response function between angular acceleration and torque measured at the interface between human body and seat. Two-order resonances of human body in the roll direction at 1 Hz and 2.5 Hz were identified. Subsequently, vertical vibrations were combined with the roll ones to investigate the human response under dual-axis roll-vertical vibration. Frequency response function analysis and partial coherence analysis were conducted, and the coupling between the vertical and roll responses of human body was discussed. The study revealed that roll vibration had minimal influences on the dynamic response of seated human body in the vertical direction within 0−10 Hz. Similarly, vertical vibration had negligible impact on human response in the roll direction around the first two resonance frequencies, including modal frequencies and associated modulus of roll apparent inertia. Finally, a dynamic model was established to represent the biodynamic characteristics of human body under combined roll and vertical vibration, and dynamic parameters of the model were identified.