Abstract: The present research included two experiments in which orthogonal designwith two covariates controlled were used. Effect of seven factors were in-vestigated: type of stimulus, complexity of stimulus, intensity of stimulus, pre-senting time of stimulus, subjects' criteria on judging the two stimuli, thenumber of the same modality, and gender——on time succession threshold.In experiment one, the results showed that four factors——the type of stimu-lus, the presenting time of stimulus, subjects' criteria on judging the twostimuli, and the number of the same modality——had significant effects onthe time succession threshold, whereas the other three factors——the complexi-ty of stimulus, the intensity of stimulus, and the gender——did not significan-tly affect the time succession threshold. In experiment two, the results showedthat there was no interaction among the four significant factors.

Key words: time, threshold, succession, covariate, orthogonal design