A power tool comprising a transmission mechanism, an output shaft driven by the transmission mechanism, and a hydraulic unit filled with a working fluid wherein, the hydraulic unit is disposed coaxially with the output shaft, and defines a rotatively driving mechanism, a first fluid chamber, and a second fluid chamber; the first fluid chamber is in intermittently fluid connection with the second fluid chamber via a fluid channel; and a relative rotation between the hydraulic unit and the output shaft causes the rotatively driving mechanism to axially compress or expand a volume of the second fluid chamber for raising or decreasing a fluid pressure of the second fluid chamber, generating a pressure difference between the first fluid chamber and the second fluid chamber to cause an instantaneous torque to the output shaft, characterized in that: the rotatively driving mechanism comprises an axially extending body protruding from an interior surface of the hydraulic unit; and during the relative rotation between the hydraulic unit and the output shaft, the axially extending body pushes a first end wall of an axially moveable member which defining one edge of the second fluid chamber towards a second end wall which defining another edge of the second fluid chamber downwards at a first position, until the second fluid chamber is decreased to a minimum volume, and wherein the first fluid chamber, the second fluid chamber and the rotatively driving mechanism are disposed inside a generally cylindrical case which is coaxial with the output shaft, the first fluid chamber surrounded by an outer side surface of the output shaft, an inner side surface of the case, and both end surfaces of the case, wherein the second fluid chamber is surrounded by an end surface of a rolling member, the first end of the axially movable member, and the outer side surface of the output shaft, and the inner side surface of the case. the hydraulic unit is disposed coaxially with the output shaft, and defines a rotatively driving mechanism, a first fluid chamber, and a second fluid chamber; the first fluid chamber is in intermittently fluid connection with the second fluid chamber via a fluid channel; and a relative rotation between the hydraulic unit and the output shaft causes the rotatively driving mechanism to axially compress or expand a volume of the second fluid chamber for raising or decreasing a fluid pressure of the second fluid chamber, generating a pressure difference between the first fluid chamber and the second fluid chamber to cause an instantaneous torque to the output shaft, characterized in that: the rotatively driving mechanism comprises an axially extending body protruding from an interior surface of the hydraulic unit; and during the relative rotation between the hydraulic unit and the output shaft, the axially extending body pushes a first end wall of an axially moveable member which defining one edge of the second fluid chamber towards a second end wall which defining another edge of the second fluid chamber downwards at a first position, until the second fluid chamber is decreased to a minimum volume, and wherein the first fluid chamber, the second fluid chamber and the rotatively driving mechanism are disposed inside a generally cylindrical case which is coaxial with the output shaft, the first fluid chamber surrounded by an outer side surface of the output shaft, an inner side surface of the case, and both end surfaces of the case, wherein the second fluid chamber is surrounded by an end surface of a rolling member, the first end of the axially movable member, and the outer side surface of the output shaft, and the inner side surface of the case. the rotatively driving mechanism comprises an axially extending body protruding from an interior surface of the hydraulic unit; and during the relative rotation between the hydraulic unit and the output shaft, the axially extending body pushes a first end wall of an axially moveable member which defining one edge of the second fluid chamber towards a second end wall which defining another edge of the second fluid chamber downwards at a first position, until the second fluid chamber is decreased to a minimum volume, and wherein the first fluid chamber, the second fluid chamber and the rotatively driving mechanism are disposed inside a generally cylindrical case which is coaxial with the output shaft, the first fluid chamber surrounded by an outer side surface of the output shaft, an inner side surface of the case, and both end surfaces of the case, wherein the second fluid chamber is surrounded by an end surface of a rolling member, the first end of the axially movable member, and the outer side surface of the output shaft, and the inner side surface of the case.