This　paper　presents　a　new　method　to　simulate　non-linear　breast　motion　by　using　a　three-dimensional　(3D)　dynamic　finite　element　model　(FEM).　The　model　consists　of　a　thorax　with　two　breasts　and　three　skin　layers　with　specific　mechanical　properties.　Using　free　breast　vibration,　the　viscous　damping　ratios　were　ascertained　to　be　0.215　for　an　80B　size　breast.　The　shear　modulus　for　the　breast　was　derived　as　the　value　that　gave　the　minimum　difference　between　the　FEM-predicted　results　and　the　experimental　data.　A　hyper-elastic　neo-Hookean　material　model　simulated　the　large　deformation　of　breast　tissue.　The　mode　shapes　of　breast　motions　at　different　natural　frequencies　were　established.　The　highest　breast　displacement　amplitude　ratio　relative　to　the　thorax　was　at　4Hz.　The　study　showed　that　FEM　can　predict　breast　displacement　with　sufficient　accuracy　and　thereby　provide　the　basis　by　which　bras　may　be　engineered　more　ergonomically　in　the　future.　Practitioner　Summary:　To　facilitate　a　theoretical　analysis　of　breast　motion　to　enable　the　design　of　more　supportive　bras,　a　dynamic　FEM　based　on　reliable　non-linear　properties　of　breast　tissues　has　been　developed.　The　methods　and　findings　have　potential　widespread　benefit　for　developing　new　products　to　promote　women＇s　health　and　comfort.