Using　the　first-principle　phonon　calculations　under　the　quasiharmonic　approximation,　thermal　expansions　in　III-nitrides　with　wurtzite　AlN,　GaN,　and　InN　are　reported.　The　results　showed　that　it　is　different　for　each　thermal　expansion　of　three　III-nitrides　at　low　temperatures,　which　is　consistent　with　their　Gruneisen　parameters　as　the　function　of　temperature.　Below　50　K,　negative　thermal　expansions　occur　in　InN,　while　GaN　and　AlN　follow　the　rule　of　positive　thermal　expansion.　To　seek　the　origin　of　positive/negative　thermal　expansion　distinction,　the　mode　Gruneisen　parameters　and　the　phonon　spectra　are　investigated.　They　indicate　that　different　low-frequency　phonon　vibration　modes　correspond　to　the　change　of　thermal　expansions.　Below　5　THz,　the　significant　weighted　negative　values　of　mode　Gruneisen　parameters,　caused　by　the　weakening　of　mixing-mode　constituted　with　two　transverse　acoustic　(TA)　modes　and　a　small　overlapped　part　of　optical　modes,　directly　lead　to　the　negative　thermal　expansion　at　low　temperatures.　(C)　2011　American　Institute　of　Physics.　[doi:10.1063/1.3627237]