It has been broadly accepted that root gap during welding continues to open in case of butt-welding without restraints. So, the restraints, such as strong back and tack weld, are essential countermeasures to prevent root gap from opening. In order to improve manufacturing sufficiency, active control of root gap behavior during welding even becomes significant. In this study, the root gap behavior during in-process control welding by additional cooling is discussed based on measurement experiments and numerical analyses. As the results, the root gap behavior during butt-welding is possible to be considerably affected by temperature distribution during in-process control welding. The cooling position and cooling intensity of controllable factors in additional cooling method have a significant influence on the root gap behavior during welding. In addition, the root gap can be closed due to rapidly cooled temperature distribution by additional cooling with appropriate cooling position. The appropriate cooling position is dependent on the mechanical melting length behind welding torch. It is significant to consider the effect of weld shrinkage during rapid cooling for closing of the root gap. It is concluded that root gap behavior during butt-welding can be controlled by the optimization of temperature distribution due to additional cooling.