Systematic　study　on　the　electronic/geometrical　structures　and　the　parity　alternation　effect　of　hydrogen-doped　cationic　carbon　clusters　CnH2+　(n　=　3-10)　have　been　carried　out　at　the　density　functional　theory　level.　The　linear　isomers　of　the　clusters　have　been　defined　as　the　ground-state　(G-S)　isomers.　The　carbon　chain　with　even-n　is　quasi-polyacetylene-like　in　configuration;　whereas　when　n　is　odd,　the　carbon　chain　displays　a　quasi-polyacetylene-like　structure　that　fades　into　a　cumulenic-like　arrangement　towards　the　center　of　the　chain.　The　even-n　cations　are　more　stable　than　the　odd-n　ones.　Theoretical　explanations　for　the　trend　of　even/odd　alternation　have　provided　based　on　a　series　of　properties　of　the　systems.　The　findings　accord　with　the　relative　abundance　of　CnH2+　species　as　recorded　in　mass　spectrometric　investigations.　(C)　2014　Elsevier　B.V.　All　rights　reserved.