Abstract: Two experiments were reported in this paper, The main experiment was intended to reveal what criteria were adopted to determine the correctness of conclusion derived from syllogistic reasoning. It was generalized that during the syllogistic reasoning, when the correctness of optional conclusion had to be decided, two determining criteria, that is, 'form criterion' and' content criterion', can be derived from the knowledge structure of any person involved,and which criterion would make more effect also depends on the reasoner's knowlege structure.

Key words: syllogistic reasoning, form determining criterion, content determining criterion