A chain saw, comprising: a housing; a guiding plate installed on the housing, such that it is movable forwardly and backwardly relative to the housing; a chain arranged around the guiding plate; a power system installed on the housing and driving the chain; and a tensioning component (5; 6; 7) comprising a knob (51; 61; 71), a driving member (52; 62; 72) and a transmission assembly (53; 63; 73) arranged between the knob (51; 61; 71) and the driving member (52; 62; 72); wherein, when the chain is in a loose state; the knob (51; 61; 71) is arranged to rotate and drive the transmission assembly (53; 63; 73) to rotate, which transmission assembly (53; 63; 73) is arranged to drive the driving member (52; 62; 72) to rotate, and the driving member (52; 62; 72) is arranged to drive the guiding plate to move to tension the chain; when the chain is in an extension state, the knob (51; 61; 71) is arranged to rotate, such that the driving member (52; 62; 72) stays still and skidding is generated inside the tensioning component (5; 6; 7); wherein the transmission assembly comprises a bracket and a plurality of elastic modules, the bracket comprises a plurality of holes for accommodating the plurality of elastic modules respectively, the elastic modules comprise an elastic member and a ball; wherein the knob comprises an end wall; characterised in that the holes are through holes and in that the end wall has a plurality of inner teeth arranged on an inner surface thereof, a positioning groove is arranged between two adjacent inner teeth, and wherein the ball is arranged to be pushable by the elastic member to latch with the positioning groove. a housing; a guiding plate installed on the housing, such that it is movable forwardly and backwardly relative to the housing; a chain arranged around the guiding plate; a power system installed on the housing and driving the chain; and a tensioning component (5; 6; 7) comprising a knob (51; 61; 71), a driving member (52; 62; 72) and a transmission assembly (53; 63; 73) arranged between the knob (51; 61; 71) and the driving member (52; 62; 72); wherein, when the chain is in a loose state; the knob (51; 61; 71) is arranged to rotate and drive the transmission assembly (53; 63; 73) to rotate, which transmission assembly (53; 63; 73) is arranged to drive the driving member (52; 62; 72) to rotate, and the driving member (52; 62; 72) is arranged to drive the guiding plate to move to tension the chain; when the chain is in an extension state, the knob (51; 61; 71) is arranged to rotate, such that the driving member (52; 62; 72) stays still and skidding is generated inside the tensioning component (5; 6; 7); wherein the transmission assembly comprises a bracket and a plurality of elastic modules, the bracket comprises a plurality of holes for accommodating the plurality of elastic modules respectively, the elastic modules comprise an elastic member and a ball; wherein the knob comprises an end wall; characterised in that the holes are through holes and in that the end wall has a plurality of inner teeth arranged on an inner surface thereof, a positioning groove is arranged between two adjacent inner teeth, and wherein the ball is arranged to be pushable by the elastic member to latch with the positioning groove.