Abstract: The illusion in prepositional and syllogistic reasoning was studied. Sixty Jinan university students participated in 2 experiments, we used the double-premises problems heading “only one of the premises is true”. The materials were prepositional in Experiment 1 and dramatized in Experiment 2. The t test showed that the participants considered the truth of each premise in turn, but neglected the fact that when one premise was true, the other was false. The result consisted with the “principle of true” predicting by mental model theory. A conversion-mechanism provided a complemented explanation for the model theory

Key words: deductive reasoning, mental model, illusion, conversion-mechanism