The　quadtree　is　one　of　the　most　advanced　techniques　contributing　to　the　excellent　compression　performance　of　high　efficiency　video　coding　(HEVC).　However,　the　computational　complexity　increases　because　the　quadtree　examines　all　coding　unit　(CU)　sizes　to　obtain　the　optimal　CU　partitioning.　This　paper　focuses　on　quadtree　degeneration　based　on　a　proposed　quadtree　probability　mechanism.　Two　techniques,　a　quadtree　probability　model　(QPM)　procedure　and　a　quadtree　probability　update　(QPU)　procedure,　are　proposed.　The　QPM　process　estimates　a　CU　distribution　model　based　on　a　quantization　parameter　(QP)　and　a　group　of　pictures　(GOP).　Based　on　the　model,　a　new　quadtree　is　constructed　by　skipping　low　probability　tree　nodes.　The　QPU　process　is　performed　to　update　the　new　quadtree　based　on　scene　content　change.　Update　addresses　model　distortion　and　ensures　the　accuracy　of　the　new　quadtree.　Experimental　results　demonstrate　that　the　proposed　quadtree　probability　mechanism　for　quadtree　degeneration　considerably　reduces　average　encoding　time　(27.55%)　for　the　low　delay　condition.　Applied　to　lossless　coding,　the　proposed　mechanism　achieves　a　significant　43.10%　encoding　time　reduction.　The　experiments　also　show　that　the　proposed　quadtree　probability　mechanism　improves　HEVC　coding　efficiency　for　a　variety　of　applications　and　sequence　characteristics.