The purpose of the study is to investigate the problem-solving strategies of children in fourth-grade, fifth-grade and sixth-grade. The investigators tried to guide children through a variety of problem-solving strategies that were in written, figural, or symbolic forms. Then the investigators examined the variation of children¡¦s spatial concepts by analyzing the problem-solving strategies used in calculating the volumes of various solids. In order to improve the teaching materials, the investigator identified the performance of spatial concepts by referring to QCAI (QUASAR Cognitive Assessment Instrument). For the purpose of designing future lessons, the investigator examined the relationship between children¡¦s problem-solving strategies level and the performance in the seven content areas in the curriculum. There are two results of this study: The first result is on the problem-solving strategies found within the same grade (4th; 5th; and 6th) and there are 3 findings. First, strategies of fourth-graders are focused on low-levels and middle-levels; strategies of fifth-graders focused on middle-level geometric space concept, and finally, strategies of sixth-grades students focused on middle-levels and high-levels. Second, there are no significant differences relating to gender. Third, the higher the grade of children is the higher the levels of their spatial concepts. In addition, the second result is the relationship between strategy levels and performance in seven content areas: revealing position corrections, and having mathematics as the most significant, and the rest in order: social studies, language arts, science and technology, arts and humanities, integrative activities, health and physical education.