In the first experiment using typical AB Paradigm, it was found, when two stimuli (T & P respectively) were serially presented without distracter between them, that their pattern of RT matched the prediction of the Parallel Model exactly. However, in the second experiment, when T was immediately followed by a backward mask, T & P’s pattern of RT accorded with the Serial Model. These results indicated that serially presented stimuli without distracter between them can be processed simultaneously. And the parallel mechanism can only deal with two objects simultaneously. Moreover, it can be interrupted by long interval between two targets.