The purpose of this study was to investigate the way children make a plan when attempting to solve an "ill-structured" problem. 4-, 5-, 6-, and 8-year-old children were requested to build something like a stair-case with small blocks so a doll can walk up a big block. The developmental trends of behavior in the preparing process, in the building process, and in the judging process were analyzed. The main results were as follows: 1)by the age of 6, children became able to orient their operations according to the conditions of the problem, 2) above the age of 6, their operation began to be internalyzed and took shape, 3)when the orientation was completed, a basic unit of operations was internalyzed. As compared with the case of "well-structured" problem solving, orientation may play a more important role in planning.