Auditory stream segregation is know as a process whereby sound elements are separated and integrated into some perceptual objects as a coherent whole, in the auditory scene analysis. Such a perceptual faculty is considered to be dependent on some factors in sounds such as similarity, good continuation, common fate, disjoint allocation, closure, and so on. For example, when we listen to fast alternative sinusoidal tone sequences, a number of separate auditory objects may be perceived, according to their acoustical attributes. It is known that such perception, fusion or fission, has close relation to frequency difference, time pattern and harmonic components in sounds. In this paper, we investigate an effect of amplitude modulation on sequential stream segregation for two close sinusoidal tones in frequency domain. As a result, it is shown that amplitude modulation improves the sequential stream segregation performance.