We determined the phase boundary of the Fddd phase in the phase diagram of polystyrene-block-polyisoprene (SI) diblock copolymer by using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). We prepared eight SI diblock copolymers having different volume fractions of polyisoprene, f(PI), in the range of 0.627 <= f(PI) <= 0.653 by living anionic polymerization. At the lowest f(PI), 0.627, SI diblock copolymer showed only the disorder-lamellar (L) transition without any order-order transitions. SI diblock copolymer having the highest f(PI), 0.653, exhibited disorder-gyroid (G)-L transitions with decreasing temperature but did not have the Fddd structure. Except for two samples described above, SI diblock copolymers exhibited disorder-G-Fddd-L transitions with decreasing temperature. It was found that the location of the Fddd region agreed with those obtained by using self-consistent-field theory calculation and Ginzburg-Landau theory.