Seat　transmissibility　is　associated　with　both　seating　dynamics　and　biodynamics.　The　aim　of　this　experiment　was　to　investigate　how　seat　transmissibilities　are　affected　by　the　horizontal　excitation　variations　as　well　as　the　presence　of　the　vertical　excitation.　Twelve　participants　were　evaluated　for　the　seat　transmissibilities　during　the　horizontal　random　excitation　at　three　amplitudes　with　and　without　additional　vertical　excitation.　The　primary　resonant　frequencies　of　the　vertical　cross-axis　and　horizontal　in-line　seat　transmissibilities　reduced　with　an　increase　in　the　horizontal　excitation.　The　seat　transmissibility　acquired　from　single-axis　excitation　exhibited　the　same　resonant　behaviour　as　those　obtained　from　dual-axis　excitation.　The　nonlinearity　of　the　seat　transmissibility　with　dual-axis　excitation　suggests　coupled　modes　are　induced　by　the　cross-axis　effect.