Low-cost　sodium-ion　batteries　have　demonstrated　great　prospects　in　energy　storage,　among　which　layered　transition　metal　oxides　hold　great　potential　as　a　cathode　material.　However,　the　notorious　phase　transition　in　layered　cathode　materials　has　greatly　hampered　their　cycle　life　due　to　large　volume　changes　upon　desodiation/sodiation.　In　this　study,　by　adopting　an　O3-type　NaNi1/3Fe1/3Mn1/3O2　(NFM)　with　controlled　synthesis　temperatures,　we　have　revealed　that　the　grain　size　is　closely　related　to　its　phase　transition　behaviors.　The　layered　material　with　a　smaller　grain　size　and　more　distorted　lattice　tends　to　experience　a　shorter　plateau　of　the　O3-P3-O3　phase　transitions　during　the　charge/discharge　process.　Despite　having　a　lower　nominal　discharge　capacity　without　the　phase　transition　plateau,　its　cycling　stability　increases　from　77.4%　to　96.2%　after　100　cycles　with　greatly　reduced　intragranular　cracks.　The　smaller　grain　size　and　lattice　distortion　act　as　a　barrier　that　prevents　the　smooth　layer　from　gliding　upon　sodium　intercalation　and　deintercalation.　This　study　focuses　on　the　influence　of　grain　size　on　battery　cycle　stability　and　provides　a　basis　for　future　analysis　of　the　structural　instability　of　layered　materials.