A reaction database system on Internet is described. The main feature lies in the representation of reaction schemes, where the mapping between atoms of a reactant and a product is preserved as the information of a reaction, enabling the classification of atoms from the viewpoint of chemical reaction. As a result, the reaction sites on a reactant and a product are extracted hierarchically based on the classification of atoms. It allows the retrieval from the point of chemical reaction as well as simple substructure searching. The system is implemented using Java as a client-server system on Internet, and is opened to the public aiming at both retrieval and update via Internet.