This　work　investigates　the　evolution　of　the　crystal　structure　of　the　topologically　closed　packed　orthorhombic　P　phase　precipitate　in　a　Re-containing　Ni-based　single　crystal　superalloy　during　thermal　exposure.　The　P　phase　is　formed　with　a　thin　needle　morphology.　The　precipitate　is　formed　from　the　matrix　with　an　initial　complex　atomic　arrangement　which　continues　to　evolve　during　the　process　of　thermal　exposure.　Based　on　the　experimental　evidence　of　this　study　and　the　theoretically　predicted　structure　reported　in　the　literature,　a　mechanism　of　the　structural　transformation　is　proposed.　The　initial　structure　is　composed　of　a　parallelogram　(P)　atomic　arrangement　configuration,　which　gradually　evolve　into　a　rectangle　(R)　atomic　arrangement　configuration　in　the　[100](P)　projection.　In　the　[010]　projection,　the　initial　structure　is　composed　of　alternating　rows　of　a　larger　parallelogram　(P＇)　and　a　larger　rectangle　(R＇)　configurations,　which　gradually　evolve　into　an　intricate　structure　of　P＇-R＇　along　the　length　[001](P)　direction　and　P＇P＇-R＇R＇　along　the　transverse　[100](P)　direction.　The　initial　structure　is　formed　for　its　structural　similarities　to　the　gamma　phase　to　minimize　lattice　mismatch.　The　final　structure　is　evolved　over　time　to　conform　to　its　thermodynamically　more　stable　state.　The　structural　evolution　is　achieved　by　collective　atomic　shuffling.　These　intricate　atomic　arrangements　gives　rise　to　the　very　faint　and　highly　dense　parallel　striation　lines　along　the　transverse　[010](P)　direction　in　[100](P)　view　and　along　the　length　[010](P)　direction　in　[010](P)　view　under　transmission　electron　microscopy　observation.　Crown　Copyright　(C)　2019　Published　by　Elsevier　Ltd　on　behalf　of　Acta　Materialia　Inc.　All　rights　reserved.