The purpose of this study was to investigate between the children`s cognitive levels the degree of the understanding of elementary current and magnetic field and molecular concepts with three levels of cognitive complexity. In this study finding were as follow. 1. The intellectual level of most 6th elementary school student in Korea, in general, stays in-the concrete operational stage some pupil have reached formal operational stage and none differential by sex. 2. The degree of understanding of elementary current and magnetic field and molecular concepts showed difficult of primary science content level. In conclusion, the understanding and application of Piaget`s theory suggest a means of facilitating a better curriculum material. Also to understand of cognitive comprehension difference with cognitive complexity was facilitates a more accurate analysis for educational purpose.