We investigated the Fddd phase in the polystyrene (PS) rich region or f(PI) < 0.5 of the phase diagram for polystyrene-block-polyisoprene (SI) diblock copolymers by using small-angle X-ray scattering and transmission electron microscope, where f(PI) is a volume fraction of polyisoprene (PI) in a SI diblock copolymer. Five SI diblock copolymers which have different f(PI) values were prepared, in the range of 0.369 < f(PI) < 0.392 by living anionic polymerization. We found the Fddd phase as an equilibrium phase in the range of 0.370 <= f(PI) <= 0.373 and 19.5 < chi N < 21.1 where chi and N are, respectively, the Flory-Huggins interaction parameter between PS and PI, and polymerization index of the diblock copolymer. The Fddd phase boundary in PS-rich region is much narrower than that in PI-rich region or f(PI) > 0.5 in terms of f(PI) and chi N. The asymmetry of the Fddd region between PS-rich and PI-rich region can be explained by the conformational asymmetry between PS and PI chains.