The purpose of this study was to investigate the relationships between children's verbal ability and their performance on Piagetian conservation tasks. The acquisition of conservation concept is a typical criterion used to make a distinction between preoperational stage and concrete operational stage in Piaget's cognitive theory of child development. As several researchers have noted, the conservation behavior of children often appears earlier in nonverbal tasks than in Piagetian tasks. It seems that children's ability to understand verbal instructions in the Piagetian test of conservation has an effect on their performance of that test. The following two hypotheses were formulated from this point of view. H.1 : There will be a positive correlation between children's verbal ability and their performance on Piagetian conservation tasks . H.2: There will be a significant difference in the children's success-failure ratio between Piagetian conservation tasks and nonverbal conservation tasks. The sample of this study was 70 six years old and 80 seven years old children drawn from a primary school located in Kongju city. Conservation tests, a Piagetian type and a nonverbal type, were developed by the authors in order to evaluate children's acquisition of the number and length conservation concepts. A linguistic competence test was also made by the researchers to measure children's ability to understand verbal instructions in the Piagetian test of conservation. All the children were tested individually. The major findings were as follows: A positive correlation was found between children's verbal ability and their success or failure in Piagetian conservation tasks. The children who succeeded in Piagetian conservation tasks were on the higher level in verbal ability than the children failed in those tasks. The difference in the means of linguistic competence scores between the two groups of children was found to be statistically significant even after taking into account initial differences in their intelligence. It was also found that the children who failed in Piagetian tasks were, for the most part, successful in nonverbal tasks. These results were used to support the hypotheses of this study. On the basis of the findings, the following conclusions can be made : First, there are positive correlations between children's verbal ability and their performance on Piagetian conservation tasks. Second, there is a possibility that children who fail in Piagetian conservation tasks may succeed in nonverbal conservation tasks. Third, one of the major reasons why children fail in the Piagetian test of conservation is their incomplete understanding of the verbal instructions. This research had some limitations. The validity and reliability of the instruments used in this study were not fully checked. The sample of the research was small and non-representative. Therefore, it should be cautious not to generalize the above conclusions widely.