In this study, a styrene/isoprene/styrene (SIS) triblock copolymer was synthesized in an intermeshing, co-rotating, twin-screw extruder. N-butyl lithium was used as the initiator. The styrene and isoprene monomer as well as 1,2-dibromoethane were sequentially added into the extruder. The weight content of isoprene in this copolymer was above 60%. Gel permeation chromatography (GPC) results showed that the polystyrene in the copolymer chains was mainly comprised of only one long block. Dynamic mechanical analysis (DMA) and transmission electron microscopy (TEM) showed that the polystyrene phase transferred from larger numbers of small microdomains to a continuous aggregation after the coupling reaction. As a result, the tensile stress and ultimate elongation was significantly improved, similar to the commercial product. Traditionally, SIS has only been synthesized by solution polymerization. The present work offers the capacity to synthesize it with very little or no solvent. This method fits the environmentally friendly trend.