Instantaneous　modal　parameters　are　effective　indicators　of　nonlinearity　in　an　aeroelastic　system.　The　Identification　of　them　requires　reliable　algorithms.　A　frequency　modulated　empirical　mode　decomposition　(FM-EMD)　method　is　proposed　in　this　paper　to　track　the　variations　in　the　modal　parameters　of　a　multi-DOF　bridge-air　system.　The　proposed　method　is　a　lifted　version　of　the　traditional　EMD　method.　It　overcomes　one　of　the　major　shortcomings　of　its　traditional　version,　i.e.　the　inability　to　separate　closely　spaced　modes.　Numerical　verification　was　provided　to　show　the　effectiveness　of　the　FM-EMD　method.　Wind　tunnel　testing　data　of　a　partially　streamlined　box　girder　sectional　model　of　a　suspension　bridge　was　processed　subsequently　with　the　proposed　method.　Instantaneous　frequencies　and　damping　ratios　of　the　aeroelastic　system　during　the　free　decay　vibration　were　realized.　Nonlinearity　in　the　transient　aeroelastic　vibration　was　identified.　(C)　2011　Elsevier　Ltd.　All　rights　reserved.