We investigate the formation of the ordered bicontinuous structures in a stereoregular diblock copolymer, isotactic polypropylene-block-polystyrene (iPP-b-PS), in which the minority PP block possessed isotactic configuration. This diblock displayed the conventional ordered bicontinuous double gyroid (OBDG) morphology upon heating above the crystal melting point of the iPP block from the as-cast state. The OBDG phase remained stable in the heating process up to the order-disorder transition. In the subsequent cooling process from the nearly disordered state, the OBDG phase first developed, but when the temperature was sufficiently low, an order-order transition from OBDG to the ordered bicontinuous double diamond (OBDD) phase occurred, and OBDD eventually became the dominant structure. The results attested that OBDD and OBDG represented the thermodynamically stable structure at the lower and the higher temperature, respectively, and the OBDG morphology formed in the as-cast state was metastable. The present finding along with that of the syndiotactic polypropylene-block-polystyrene (sPP-b-PS) reported previously consolidated the role of configurational regularity in stabilizing the otherwise unstable OBDD phase for diblock copolymers. The stability of the OBDD structure was attributed to the cooperative effect of the relatively high polydispersity and the helical segment formation of the stereoregular minority block, as the conformational entropy loss arising from the packing frustration of the minority block in the network domain was effectively compensated by the release of enthalpy via the formation of helical segments and their associations.