The present study is to investigate whether the presentation of imformation with objective uncertainty arouses conceptual conflict and cognitive motivation and there is an optimal level of objective uncertainty both on conceptual conflict and on cognitive motivation, Objective uncertainty is manipulated by the number of alternatives. The first of the three experimental groups is presented with two alternatives, the second with four alternatives, the third with six alternatives, while the control group is presented with only one. The alternatives used are about the theories of evolution as Darwinism, Lamarkism and so on. The hypotheses tested are as follows: 1) There is an optimal level of odjective uncertainty, which arouses a maximal conceptual conflict and cognitive motivation. 2) When objective uncertainty arouses the optimal conceptual conflict, then an individual finds pleasure. Therefore, when conceptual conflict below the optimal level is aroused in him, he will seek for higher level of uncertainty. 3) This optimal level of students with high imformation-processing ability will reach higher point than that of students with low ability. In experiment I 144 junior high school pupils are divided into the three experimental groups and one control group. In experiment II 72 junior high school pupils were divided into the twoor fouralternative group. In experiment III 144 senior high school students were divided into the three experimental groups and one control group. Each of the three experiments consisted of the two sessions ; in the first session, different imformation concerning the level of objective uncertainty was presented to each of the four groups, and the Ss of the three experimental groups were asked to select an alternative which they thought to be correct. Then conceptual conflict and cognitive motivation were measured. In the second session, the imformation to reduce conceptual conflict was given, and retention of this imformation as an index of motivation, was investigated. The index of conceptual conflict was the degree to which Ss were puzzled in deciding the alternative. The indexes of cognitive motivation were as follows: 1) The degree of epistemic curiosity, or the degree to which they wanted to know about the correct alternative, and the degree to which they liked to listen to the rest parts of the story. 2) The number and content of epistemic questions as possible about the experimental content. 3) The degree of imformation gathering ; Ss were given a booklet on the theories of evolution and were asked to read it in a limited time. 4) Retention of imformation to reduce conceptual conflict. 5) Approach tendency toward uncertainty, or the degree to which they wanted to know more about other theories of evolution. The results obtained were as follows: In experiment I and II, it was verified that there was an optimal level of objective uncertainty on conceptual conflict, that is, conceptual conflict was the highest in the four-alternative level. Concerning motivation there was an optimal level, but which differed with the indexes ; the four-alternative level was the optimal on epistemic curiosity and imformation gathering, while on epistemic questions and retention of information the two-alternative level was the optimal. The two-alternative level showed the approach tendency toward uncertainty. In experiment III, through all indexes of conceptual conflict and cognitive motivation, the fouraltarnative level was the optimal. Especially both on epistemic questions and retention of imformation, we found increase of the optimal level, which verified the hypothese 3.