The method of Line Heating is mainly applied to the shipbuilding industry. It is very effective to releasse the strain of welded structure blocks, and now we began to apply it to bend all sorts of plate into arbitrary curvature. The mechanism of bending plate by Line Heating is thought as follows ; 1st step : Formation of plastic zone on the heating line from the expansion of the part of the line. 2nd step : Shrinkage of the heated zone and appearance of tensile stress by the existence of the plastic zone. The worked plate is bended by the tensile stress of 2nd step. The final purpose of applying this method is to have the ability of producing an arbitrary curvature with the most economical working speed. But, it is very difficult to do it because of difficulty of controlling the working conditions. Line Heating is also as well as any other gas working method very sensitive to the variance of the working conditions. So, at first we must examine the sensitivity of Line Heating Method to every factor, which constitutes the working condition. In this experiment we picked up 7 factors which constitute the working condition. They are tip height of burner, thickness of test piece, pressure of oxygen, torch angle to the test piece, diametre of burner nozzle, volume of cooling water and cooling method. We used the method of experimental design, orthogonal array L₃₂ (2³¹), to this experiment. From this experiment we got the main effects of tip height, thickness of test piece, pressure of oxygen, and cooling method. We could not observe the interactions between tip height and pressure of oxygen and between thickness of test piece and pressure of oxygen. We could not observe the main effects of torch angle, diametre of burner and volume of cooling water. From these results only, we may say that the method of Line Heating is most sensitive to the thickness of plate, secondarily to the tip height of burner, thirdly to method of cooling.