Efficient　light　harvesting　was　observed　over　CdS　photodeposited　on　In2O3　photonic　crystals　during　the　photocatalytic　hydrogenation　of　4-nitroaniline　to　p-phenylenediamine.　The　highest　conversion　of　4-nitroaniline　and　selectivity　of　p-phenylenediamine　over　the　In2O3　photonic　crystal　supported　CdS　were　similar　to　93%　and　similar　to　99%,　respectively,　which　were　better　than　that　achieved　for　commercial　hexagonal　CdS.　The　existence　of　the　photonic　crystal　structure　was　responsible　for　the　higher　efficiency　of　In2O3　photonic　crystal　supported　CdS.　The　ordered　structure　facilitated　the　mass　transport.　The　elaborate　tuning　of　the　photonic　band　gap　activated　the　slow　photon　enhancement　effect　on　the　blue　edge　for　intensifying　the　light　harvesting　efficiency　of　CdS.　Moreover,　CdS　supported　on　In2O3　photonic　crystals　exhibited　higher　photocatalytic　stability　than　that　on　the　In2O3　nanocrystals.　The　mechanism　of　photocatalytic　hydrogenation　over　In2O3　photonic　crystal　supported　CdS　was　discussed.　Our　results　provided　guidance　for　efficiently　utilizing　light　for　photocatalysis　by　applying　photonic　crystals　as　support.