In order to examine the characteristics of thinking procedure of underachievers, three tests, Grid, Objective Sorting and dumber Strategy were given tob4pairs of achievers and under-achievers of 5th-, Eth- and 7th- grades in public schools in Austin, Texas, U.S.A. The subjects were matched in sex, age school year, IQ level and parents' socioeconomical status. Differences between achievers and underachievers in Grid were not found m the answers themselves, but were quite significant with regard to the processes through which the answers were derived. Object Sorting 'Test, given individually by using 33 subjects, revealed the differences between underachievers and achievers in the three scores-abstract and concrete behavior, differentiation on sorting, and average number of objects assorted as a group. The differences were that achievers sort the objects with snore integrated, differentiated and abstract concepts than Underachievers do. Results of Number Strategy Test, given as a group test, explain that achievers treat numbers conceptually while underachievers do symbolically or sensorially. Froze the results of the three tests mentioned above, the author has came to the following conclusions: Underachievers are low in abstract thinking and their concept is concrete, personal and empirical, while achievers' concept is, abstract, impersonal and general. The former tend to solve problems by trial and error method based on their sensory impression, and, therefore, they seem to use their spatial and perceptual ability for solving the complicated problems. On the contrary, the latter use their reasoning ability for problem, solving with hypothetico-deductive attitude. Future problems have been proposed for the establishment of appropriate teaching methods based on the characteristics of underachievers' intellectual ability thus found.