A graded-index (GI) polymer rod was prepared by interfacial-gel polymerization, in which polymerization was performed in a PMMA tube and bromobenzene (BB) was used as a higher refractive index molecules. By using the new technology, a GI polymer rod without any defect of void or bubble was made. A detailed mechanism and simulation of graded-index formation were given according to the Vrentas-Duda free volume theory.