A method of evaluating the clamp force in a threaded joint between a bolt and a matching threaded piece during a tightening operation, which method includes the steps of: - tightening the joint by means of a torque delivering device, - registering a rotation angle (α) between the at least one bolt and the matching threaded piece during tightening of said joint, - measuring a torque provided to the joint during tightening thereof, - simultaneously inducing ultra-sonic pulses into the bolt and measuring the time of flight of said pulses in said bolt, - in response to a measured increase of the torque, monitoring the torque as a function of the rotation angle (α) in order to determine an increase rate (dT/dα) of the torque, and - based on the determined increase rate (dT/dα) of the torque, determining an initial rotation angle (α 0) of the tightening operation where the torque starts to increase as a function of bolt deformation, characterised in that the method further involves the steps of: - monitoring the time of flight (TOF 1) of the ultra-sonic pulses as a function of the rotation angle (a), in order to determine an increase rate (dTOF/dα) of the time of flight, - defining said initial rotation angle (α 0) of the tightening operation as a rotation angle where the time of flight starts to increase, - determining the total increase of the time of flight (ΔTOF) during the tightening operation based on said initial rotation angle (α 0) of the tightening operation and said increase rate (dTOF/dα) of the time of flight, and - evaluating an established clamp force in the joint based on the total increase of the time of flight (ΔTOF), the clamp force being presumed to be proportional to the total increase of the time of flight (ΔTOP) according to the equation F = k 1* ΔTOF, where k 1 is a bolt specific constant dependent on the geometry and material of the bolt. - tightening the joint by means of a torque delivering device, - registering a rotation angle (α) between the at least one bolt and the matching threaded piece during tightening of said joint, - measuring a torque provided to the joint during tightening thereof, - simultaneously inducing ultra-sonic pulses into the bolt and measuring the time of flight of said pulses in said bolt, - in response to a measured increase of the torque, monitoring the torque as a function of the rotation angle (α) in order to determine an increase rate (dT/dα) of the torque, and - based on the determined increase rate (dT/dα) of the torque, determining an initial rotation angle (α 0) of the tightening operation where the torque starts to increase as a function of bolt deformation, - monitoring the time of flight (TOF 1) of the ultra-sonic pulses as a function of the rotation angle (a), in order to determine an increase rate (dTOF/dα) of the time of flight, - defining said initial rotation angle (α 0) of the tightening operation as a rotation angle where the time of flight starts to increase, - determining the total increase of the time of flight (ΔTOF) during the tightening operation based on said initial rotation angle (α 0) of the tightening operation and said increase rate (dTOF/dα) of the time of flight, and - evaluating an established clamp force in the joint based on the total increase of the time of flight (ΔTOF), the clamp force being presumed to be proportional to the total increase of the time of flight (ΔTOP) according to the equation F = k 1* ΔTOF, where k 1 is a