Abstract: This paper reported an investigation on the self-assembly behavior of styrene-isoprene-styrene triblock copolymer(SIS) in an n-hexane solvent by an elastic light scattering method. The results indicated that the peak light scattering intensity showed a concave trend with a variation of temperature, and the effective grain diameter increased from 30.1 nm to 943.4 nm with a decrease of temperature. The molecular chains of SIS presented different aggregation states in the solution at different temperature. The light scattering signal was related to the aggregation of SIS and therefore can be used to analyze the phase separation mechanism of SIS. This article provides a new method for the study of phase separation mechanism of block polymers.