Abstract: A method is provided for determining the rotational stiffness of a drill string for drilling of a borehole in an earth formation, the drill string having a bottom hole assembly (BHA) and an upper end driven by a rotational drive system. The method comprises the steps of determining the time derivative of the drill string torque during drilling of the borehole at a selected time when stick-slip of the BHA occurs, determining the nominal rotational speed of the drill string at an upper part thereof and at said selected time, and determining the rotational stiffness of the drill string from a selected relationship between said time derivative of the drill string torque and said nominal rotational speed at the upper part of the drill string.