The　highly　efficient　photocatalysis　of　semiconductors　by　controlling　the　size　of　noble　metal　co-catalysts　has　been　widely　used.　However,　it　is　unknown　whether　metallic　materials,　as　a　new　type　of　co-catalyst,　have　a　size　effect　in　this　process.　In　this　study,　by　using　CTAB　(hexadecylcetyltrimethyl-ammonium　bromide)　as　a　surfactant,　nanocomposites　of　CoS2/CdS　nanorods　(NRs)　with　different　loads　and　sizes　of　CoS2　were　fabricated　by　using　the　facile　hydrothermal　method.　The　size　of　the　metallic　CoS2　nanoparticles　(NPs)　as　co-catalysts　could　be　controlled　from　∼　9　nm　to　∼　45　nm,　and　the　size　effect　of　CoS2　NPs　was　explored　for　the　photocatalytic　evolution　of　hydrogen.　The　rate　of　production　of　photocatalytic　hydrogen　of　0.5　%　CoS2/CdS　NR　nanocomposites,　with　∼　9　nm　CoS2　NPs,　was　13　times　that　of　the　pristine　CdS　NRs　owing　to　the　maximum　difference　in　work　function　between　CdS　and　CoS2.　The　results　of　experiments　and　DFT　calculations　show　the　effective　electron　separation　and　transfer　on　the　CdS　NRs　through　the　Schottky　junction　between　CoS2　NPs　and　CdS　NRs.　Furthermore,　the　size　effect　of　metallic　materials　was　verified,　which　is　significant　for　enhancing　the　photocatalytic　activity　of　semiconductors.　©　2020　Elsevier　B.V.