An electrosurgical instrument, comprising: a housing; a shaft coupled to the housing, the shaft having a proximal end and a distal end; an end-effector assembly (100,600) disposed at the distal end of the shaft, the end-effector assembly including first and second jaw members (110,120,420,610,620), at least one of the first and second jaw members movable from a first position wherein the first and second jaw members are disposed in spaced relation relative to one another to at least a second position closer to one another wherein the first and second jaw members cooperate to grasp tissue therebetween; a vibration coupler including a distal end movably coupled to a proximal end of the second jaw member, wherein the vibration coupler is configured to impart a mechanical vibration to the second jaw member to treat tissue disposed between the first and second jaw members; and an oscillation mechanism (30,1000,1100,1200,1300) configured to generate mechanical vibration in the vibration coupler, wherein: the first jaw member (110,610) includes an electrically-conductive tissue-engaging surface; and the second jaw member (120,420,620) includes an electrically-conductive tissue-engaging surface and a protruding element (122,422,622) extending therefrom, the protruding element configured as an elongated strip extending along a length of the second jaw member; characterized in that the electrically-conductive tissue-engaging surface of the first jaw member is configured as a tissue-sealing plate facing the electrically-conductive tissue-engaging surface of the second jaw member, and the protruding element (122,422,622) is made of an electrically non-conducting material located centrally within the electrically-conductive tissue-engaging surface of the second jaw member (120,420,620) in opposing relation to a central portion of the electrically-conductive tissue-engaging surface of the first jaw member (110,610). a housing; a shaft coupled to the housing, the shaft having a proximal end and a distal end; an end-effector assembly (100,600) disposed at the distal end of the shaft, the end-effector assembly including first and second jaw members (110,120,420,610,620), at least one of the first and second jaw members movable from a first position wherein the first and second jaw members are disposed in spaced relation relative to one another to at least a second position closer to one another wherein the first and second jaw members cooperate to grasp tissue therebetween; a vibration coupler including a distal end movably coupled to a proximal end of the second jaw member, wherein the vibration coupler is configured to impart a mechanical vibration to the second jaw member to treat tissue disposed between the first and second jaw members; and an oscillation mechanism (30,1000,1100,1200,1300) configured to generate mechanical vibration in the vibration coupler, wherein: the first jaw member (110,610) includes an electrically-conductive tissue-engaging surface; and the second jaw member (120,420,620) includes an electrically-conductive tissue-engaging surface and a protruding element (122,422,622) extending therefrom, the protruding element configured as an elongated strip extending along a length of the second jaw member; the first jaw member (110,610) includes an electrically-conductive tissue-engaging surface; and the second jaw member (120,420,620) includes an electrically-conductive tissue-engaging surface and a protruding element (122,422,622) extending therefrom, the protruding element configured as an elongated strip extending along a length of the second jaw member; characterized in that the electrically-conductive tissue-engaging surface of the first jaw member is configured as a tissue-sealing plate facing the electrically-conductive tissue-engaging surface of the second jaw member, and the protruding element (122,422,622) is made of an electrically non-conducting material located centrally within the electrically-conductive tissue-engaging surface of the second jaw member (120,420,620) in opposing relation to a central portion of the electrically-conductive tissue-engaging surface of the first jaw member (110,610).