The invention refers to a method for in-line calibration of an industrial robot (1). The robot (1) comprises a fixed base section (2) and a multi chain link robot arm (3). The chain links (4) are interconnected and connected to the base section (2) of the robot (1), respectively, by means of articulated joints (5). An end effector (6) of the robot arm (3) can be moved in respect to the base section (2) within a three-dimensional workspace into any desired location. The idea is to move the end effector (6) into a predefined calibration location and to determine characteristic parameters of the robot (1) for that location. The characteristic parameters are compared to previously acquired values of the corresponding parameters for that calibration location. The differences between the characteristic parameters of the current location and the previously acquired parameters are used for correcting the kinematic model of the robot (1) and during normal operation of the robot (1) to enhance the accuracy of movement of the distal end (6). The end effector (6) is moved exactly into the calibration location by means of an iterative closed loop control process, in which light sources (7) fixedly connected to the end effector (6) emit light rays which impact on at least one optical position sensor (12) fixedly positioned in respect to the robot base (2). The end effector (6) is moved such that the actual ray positions (20) on the sensors (12) are moved to a predefined position (20') corresponding to the predefined calibration location by means of the iterative process.