3D　inverse　opal　ZnO　photonic　crystals　(ZnO-PCs)　with　designed　photonic　bandgap　(PBG)　were　prepared　to　study　the　pore　size　and　slow　photon　effect　on　photocatalytic　dye　degradation.　The　PBGs　of　these　ZnO-PC　films　were　tuned　experimentally　by　variation　of　the　polystyrene　sphere　size　of　the　opal　templates.　It　was　found　that　there　is　competition　between　the　surface　area　and　mass　transport　with　increasing　pore　size　during　photocatalysis.　ZnO-PCs　exhibited　higher　photocatalytic　activity　under　visible　light　irradiation,　when　the　probe　molecules　were　absorbed　and　well　matched　with　the　PBG.　The　enhancement　could　be　attributed　to　intensified　dye　photosensitization　as　a　result　of　the　slow　photon　effect　at　the　PBG　edges,　thus　leading　to　a　remarkable　improvement　in　the　light　trapping.　The　present　results　provide　useful　information　for　developing　high　performance　photocatalysts　and　photoanodes　based　on　artificial　photonic　crystal　design.　©　2013　The　Royal　Society　of　Chemistry.