The　commutation　failure　is　a　common　fault　in　line-commutated　converter　(LCC)　based　high-voltage　direct　current　(HVDC)　transmission　system.　The　commutation　failure　is　modeled　and　its　influence　on　reliability　of　HVDC　transmission　systems　is　studied　in　this　paper.　First,　the　commutation　failure　mechanism　of　LCC-HVDC　transmission　systems　is　analyzed.　Then,　the　commutation　failure　criterion　is　presented　based　on　the　concept　of　commutation　area.　Moreover,　a　concise　equivalent　reliability　model　of　the　commutation　failure　is　built.　The　corresponding　evaluation　method　is　proposed　based　on　Monte　Carlo　simulation.　Furthermore,　the　impacts　of　commutation　failures　on　reliability　are　studied　by　introducing　the　reliability　model　into　the　F&D　approach　in　HVDC　reliability　evaluation.　Extensive　studies　are　conducted　on　a　CIGRE　benchmark　system,　which　validate　the　presented　methods.　The　evaluation　results　show　that　the　commutation　failures　influence　the　reliability　of　HVDC　system　obviously.　After　the　consideration　of　commutation　failures,　the　energy　unavailability　of　the　HVDC　transmission　system　increases　more　than　9%.　©　2015　IEEE.