Although the idea that education should enhance the ability of students' thinking is never new, it has been recently great interest among many teachers and researchers. As an approach to learning potential, Gamlin has proposed a theory of similarity thinking in which the development of several types of thinking are described. The theory deals with the issues of the generalization of knowledge and the child's potential to learn. Also he has developed several assessment tasks for describing the development of similarity reasoning. The main purpose of this study was to validate Gamlin's theory of similarity reasoning and the new version of the Similarity Thinking Instrument(STI) in the context of young children in Korea. The STI, Raven's Coloured Progressive Matrices(CPM), Bender's Visual Motor Gestalt Test(BGT) were administered up to 113 children between three and six years old. In addition, data of teacher's evaluation on children's cognitive development were obtained. There were significant differences in performance on the STI as a function of age showing that, in general, older children were more successful than younger ones with the more difficult tasks on the STI. Correlation coefficients for the CPM, BGT, and Teacher's Rating Scale showed that performance on the STI and these measures of intellectual ability were related significantly and positively. These findings were discussed with respect to concurrent validity issues. Results also showed that the training with STI made a difference in children's performance on the STI and the CPM. This provides evidence of modifiability of cognitive skills and supports the 'dynamic' view of learning potential. The implications of the findings from this study were discussed with respect to further study.