Piaget's structural theory of formal thought suggests that a construct of formal reasoning exists. The content of the task and type of problem employed are often ignored in Piagetian based studies but are important for generalizing findings to other studies and to educational problems. Empirical relationships are explored among the theoretically different Piagetian logical operation and psychometric assessment of intelligence, and school achievement. Subjects employed in the study were 202 11th grade students (Male 100. female 102) in Seoul area. Family background of the subjects was middle class in terms of social economic atatus. Factor analysis of performance on the Piagetian Logical Operation Test. and General Intelligence Test. and School Achievement Test indicated that. Piagetian, IQ, and school achievement test overlap to some degree. but also measure different aspects of cognitive functioning. Piagetian logical operation predicts 35% of the total variation of school achievement (multiple correlation coefficien between Piagetian logical operation and school achievement was .59) while the IQ predicts 31 % of the total variation of school achievement (multiple correlation coefficient between IQ and school achievement was .56).