It is desirable to control welding distortion and to apply this procedure during welding, rather than before or after welding in order to reduce the cost. The method suggested in this paper is to cool the hot section of weld metal during the welding of a fillet T-joint, in order to control rotational distortion in the web. Using this method, increasing the gap distance between web and flange is avoided so that it is possible to continue the welding procedure. Distortion behaviour of a fillet T-joint during in-process control welding by additional cooling is numerically analysed by the three-dimensional thermal elastic-plastic analysis considering a moving heat source with weld metal deposition. It is confirmed that the additional cooling method is effective to reduce the rotational distortion, which makes welding process impossible. Under the proper condition of cooling, it is possible to weld a T-joint without any tacking or restraint.