Stable　ordered　phases　of　W2C　stem　from　the　attractiveinteraction　of　carbon　vacancies　in　L　&　PRIME;3-typelattice.　The　disordered　phase　of　W2C　is　stabilized　mainlyby　its　configuration　entropy.　Carbon　atom　migration　via　a　sequentialintralayer　mechanism　is　the　favorable　pathway　for　the　phase　transformationbetween　different　structures　of　W2C.　Tungstensubcarbide　(W2C)　is　widely　applied　to　industrialcatalysts,　military　industries,　and　aerospace　facilities　because　itpossesses　excellent　high-temperature　performance　and　superior　mechanicalproperties.　However,　contradictory　data　on　the　crystal　structure　ofW(2)C　including　its　disordered　and　different　ordered　phaseshave　been　often　reported　in　the　literature,　and　atomic-scale　understandingof　W2C　polymorphic　structures　has　not　yet　reached　a　consensus.Based　on　the　L　&　PRIME;3-type　lattice,　we　have　performedfirst-principles　calculations　to　study　the　stability　of　L　&　PRIME;3-WC,　the　interaction　of　dilute　carbon　vacancies　in　L　&　PRIME;3-WC,　the　stable　ordered　structures　of　W2C　up　to　a　unit　cell　containing　ten　formula　units,　and　the　phase　transitionamong　five　stable　ordered　W2C　structures.　Our　results　indicatethat　carbon　vacancies　in　L　&　PRIME;3-WC　can　exhibitan　attractive　interaction,　which　provides　an　essential　driving　forceto　stabilize　the　ordered　structures　of　W2C.　The　high-temperaturedisordered　&　beta;-W2C　with　L　&　PRIME;3-typelattice　is　more　likely　to　be　stabilized　by　configuration　entropy.The　stable　ordered　W2C　structures　are　all　extended　in　the　ab　plane　of　the　L　&　PRIME;3-type　latticerather　than　along　the　c　axis　and　possess　some　specificdistribution　patterns　of　aggregate　carbon　vacancies.　New　ordered　W2C　structures　with　formation　energies　lower　than　those　of　&　beta;&　DPRIME;-W2C　and　&　epsilon;-W2C　are　foundto　be　mechanically　and　dynamically　stable.　Carbon　atom　migration　betweenthe　interlayers　of　the　L　&　PRIME;3-type　lattice　viaa　sequential　mechanism　is　an　energetically　favorable　pathway　for　thephase　transition　among　different　W2C　modifications.　Ourresults　bring　a　deep　insight　into　the　understanding　of　the　stableW(2)C　polymorphic　structures　and　would　be　very　helpful　toidentify　the　ordered　structures　of　highly　nonstoichiometric　tungstencarbide　and　other　transition　metal　carbides　in　experiments.