Phase　transition　is　a　physical　phenomenon　that　attracts　great　interest　of　researchers.　Although　the　theory　of　second-order　phase　transitions　is　well-established,　their　atomic-scale　dynamics　in　polycrystalline　materials　remains　elusive.　In　this　work,　second-order　phase　transitions　in　polycrystalline　Cu2Se　at　the　transition　temperature　are　directly　observed　by　in　situ　aberration-corrected　transmission　electron　microscopy.　Phase　transitions　in　microcrystalline　Cu2Se　start　at　the　grain　boundaries　and　extend　inside　the　grains.　This　phenomenon　is　more　pronounced　in　nanosized　grains.　Analysis　of　phase　transitions　in　nanocrystalline　Cu2Se　with　different　grain　boundaries　demonstrates　that　grain　boundary　energy　dominates　unsynchronized　phase　transition　behavior.　This　suggests　that　the　energy　of　grain　boundaries　is　the　key　factor　influencing　the　energetic　barrier　for　initiation　of　phase　transition.　The　findings　advance　atomic-scale　understanding　of　second-order　phase　transitions,　which　is　crucial　for　the　control　of　this　process　in　polycrystalline　materials.