Relationship between phase morphology and optical properties of polystyrene and polyarylate (PS-PAr) block copolymers synthesized from telechelic polystyrene has been investigated. In the PS-PAr block copolymers, the PAr domains with higher melt viscosity were dispersed in the PS phase matrix with lower melt viscosity over the wide range of their composition from PS/PAr = 25/75 to 75/25 (wt ratio). The PAr domain size was dependent on the reactive ratio of PAr determined analogously by the mole fraction of the fed telechelic polystyrene. By controlling the mole fraction of the telechelic polystyrene more than 0.016 in synthesizing the PS-PAr block copolymer, the size of PAr domains could be reduced to the microscopic scale (smaller than 100 nm). Then, the PS-PAr block. copolymers exhibited almost the same transparency as PAr in spite of the large difference in the refractive index between the PS and the PAr phase. Birefringence free condition for the PS-PAr block copolymers was determined by not only the PS/PAr composition but also the balance in the degree of molecular orientation of these chains. The latter factor suggests that PS and PAr chains undergo inhomogeneous stress and relaxation history during the injection process. By controlling Mn (number average molecular weight) and weight fraction of the fed OH-PS-OH around 20 000 and 55 wt %, respectively, in the synthesis of the PS-PAr block copolymer, the PS-PAr block copolymer exhibited almost zero birefringence without any sacrifice of transparency. Because in the PS-PAr block copolymer low birefringence and high transparency can coexist by controlling the adequate feeding condition in the synthesis process, the PS-PAr block copolymer would be a promising material for optical applications, such as a substrate of optical disks or optical lenses.