With　great　potential　in　precision　medical　application,　cell　biomechanics　is　rising　as　a　hot　topic　in　biology.　Cell　nucleus,　as　the　largest　component　within　cell,　not　only　contributes　greatly　to　the　cell＇s　mechanical　behavior,　but　also　serves　as　the　most　vital　component　within　cell.　However,　cell　nucleus＇　mechanics　is　still　far　from　unambiguous　up　to　now.　In　this　paper,　we　attempted　to　characterize　and　evaluate　the　mechanical　property　of　isolated　cell　nuclei　using　Atomic　Force　Microscopy　with　a　tipless　probe.　As　indicated　from　typical　indentation,　changing　loading　rate　and　stress　relaxation　experiment　results,　cell　nuclei　showed　significant　dynamically　mechanical　property,　i.e.,　time-dependent　mechanics.　Furthermore,　through　theoretical　analysis,　finite　element　simulation　and　stress　relaxation　experiment,　the　nature　of　nucleus＇　mechanics　was　better　described　by　poroelasticity,　rather　than　viscoelasticity.　Therefore,　the　essence　of　nucleus＇　mechanics　was　clarified　to　be　poroelastic　through　a　sophisticated　analysis.　Finally,　we　estimated　the　poroelastic　parameters　for　nuclei　of　two　types　of　cells　through　a　combination　of　experimental　data　and　finite　element　simulation.　©　2016　Author(s).