Polystyrene-block-polyisoprene-block-poly (butyl acrylate) or PS-b-PI-b-PBA triblock copolymer with different ratio (5:5 and 4:6) of styrene and isoprene unit have been synthesized by anionic polymerization. The product had been characterized by using Scanning Electron Microscopy (SEM) and N-Nuclear Magnetic Resonance (H-NMR). The results of SEM images reveal that more rubber matrix can be seen for 4:6 ratios as compared to 5:5 ratios due to high content of isoprene in the copolymer. H-NMR results indicate that the peaks of polystyrene, polyisoprene and butyl acrylate for ratio 5:5 exists at 6.50 – 7.28 ppm, 4.68 – 4.88 ppm, and 0.80 – 1.83 ppm. As for ratio 4:6, the peaks can be observed at 6.30 – 7.33 ppm, 5.19 – 5.70 ppm, and 1.18 – 1.78 ppm respectively. The H-NMR analysis of the copolymer microstructure confirms that both copolymer are primarily consists of polystyrene, polyisoprene and polybutyl acrylate segment.