The purpose of Exp. 1 was to investigate the relationship between cognitive development and language development. Piaget's number and liquid conservation tests were used in order to decide the cognitive developmental stage. The comprehension, production and transformation tests of active and passive sentences were used to examine the linguistic capacities. Subjects consisted of 150 children (Boys 81, Girls 69), aged from 5 years 2 months to 6 years 2 months with a mean of 5 years 8.9 months. The results were as follows; 1) According to the performance of number and liquid conservation tasks, children were classified in three developmental stages; conservation stage (Cgroup), intermediate stage (I group), and nonconservation stage (NC group). There was no difference in the mean age among three groups. The analysis based on the conservation tests showed that the children in C group obtained high score in three language tests and the children in NC group obtained low score. 2) We classified children with the performance of three language tests to three groups; high score group, intermediate group and low score group. We found that the children who obtained high score in language tests showed the tendency to be conserver and the children obtained low score showed the tendency to be nonconserver. It was suggested that there was a close relationship between cognitive development and language development. The purpose of Exp. 2 was to find how the cognitive development and the language development influence each other. We introduced the training experiment to attain the present purpose. Subjects were children who were nonconserver and who obtained low score in transformation test. Ss were assigned to two experimental groups (operational training group, OT and language training group, LT) and control group. Each group consisted of 15 children. In OT group, children were trained in number conservation tasks mainly by verbal rule instruction of conservation principles (identity, reversibility and compensation). In LT group, children were trained in the transformation tasks by verbal rule instruction of active and passive voice transformation. The experiment consisted of pretest, training session, post test 1 and posttest 2. The results were as follows; 1) In OT group, children progressed significantly from pretest to post test 1 and post test 2 in number conservation test. However, they did not progress in language tests. 2) In LT group, children did not so much progress in language tests. They did not show the progress in number conservation test. Sinclair showed that the training of language usage was not effective for the cognitive development. She concluded that language was structured by intellectual operations. The results obtained by the training experiment suggested that her conclusion was questionable.