Controlling　the　internal　structure　of　block　copolymer　(BCP)　particles　has　a　significant　influence　on　its　functionalities.　Here,　a　structure-controlling　method　is　proposed　to　regulate　the　internal　structure　of　BCP　Janus　colloidal　particles　using　different　surfactants.　Different　microphase　separation　processes　take　place　in　two　connected　halves　of　the　Janus　particles.　An　order-order　transition　between　gyroid　and　lamellar　phases　is　observed　in　polymeric　colloids.　The　epitaxial　growth　during　the　structural　transformation　from　gyroid　to　lamellar　phase　undergoes　a　two-layered　rearrangement　to　accommodate　the　interdomain　spacing　mismatch　between　these　two　phases.　This　self-assembly　behavior　can　be　ascribed　to　the　preferential　wetting　of　BCP　chains　at　the　interface,　which　can　change　the　chain　conformation　of　different　blocks.　The　Janus　colloidal　particles　can　further　experience　a　reversible　phase　transition　by　restructuring　the　polymer　particles　under　solvent　vapor.　It　is　anticipated　that　the　new　phase　behavior　found　in　Janus　particles　can　not　only　enrich　the　self-assembly　study　of　BCPs　but　also　provide　opportunities　for　various　applications　based　on　Janus　particles　with　ordered　structures.　©　2024　Wiley-VCH　GmbH.