We report the morphology and phase behaviors of binary blends of polystyrene-block-polyisoprene (SI) copolymers, which were studied by a small-angle X-ray scattering (SAXS) technique. The neat SI copolymers employed in this study have almost the same molecular weights but different volume fractions of polystyrene block (phi(PS) = 0.26 and 0.65). Blends with various overall volume fractions of polystyrene block (<(phi(PS))over bar>), ranging from 0.41 to 0.60, were prepared from these two SI copolymers, It was found that the morphology can be controlled by adjusting <(phi(PS))over bar>. For the blends with 0.57 less than or equal to <(phi(PS))over bar> less than or equal to 0.60, the morphological transition from lamellae to gyroid phases was observed upon increasing the temperature. We present an experimentally determined phase diagram for the blend by plotting chi Z vs <(phi(PS))over bar>, where chi is the interaction parameter and Z is the reduced value of the total degree of polymerization of the block copolymer.