Abstract: This article selected one abstract proposition and three concrete propositions as the materials,studied the cognitive mechanisms and developmental traits of conditional reasoning for adolescentaged 10 to 21 years by conditional reasoning and estimation probability tasks.The results indicated that:(1)There was a distinct difference in the same syllogistic inference(MP、DA、AC、MT)of three concrete propositions and in every concrete propositions four syllogistic inferences(MP、DA、AC、MT).(2)The process of resolving conditional inference is an intuitional and effective cognitive process based on estimation of events probabilities that was determined by increasing of experiences and knowledge and improving of subjects cognitive level.It was not completely correct that judged the individual's ability of reasoning in according to logic standards.

Key words: conditional inference, conditional probability, model theory, rule theory