Abstract: The relationship of the morphology and mechanical property with the geometry structure of a set of star shaped SIS thermoplastic elastomers have been examined by means of Electron Microscopy, Tensile Tester and Rheovibron Viscoelastometer. The SIS thermoplastic elastomers were prepared by anionic polymerization using polyfunctional chlorosilane as linking agent and had the same arm length but different degree of branching. The results obtained show that the domain size of polystyrene, the tensile strength and Young's moduli of the polymers increase with increase of the degree of branching. Based on the crosslinked rubber equation and Kerner equation, the moduli expression as a function of branching degree has been derived.