The electric utility industry around the world is undergoing a revolutionary transition from vertically integrated monopoly structures to a competitive structures. Competition in an open transmission access(OTA) makes the market participants access easily the transmission system in a non-discriminatory and equitable manner. The competitions of suppliers and customers make the electric market price change every hour. This paper presents a web program in the internet environment with a function of optimal power flow(OPF) calculation. The Web program gives the nodal marginal cost and the congestion charge using the shadow prices resulted from OPF. This web program is realized by a Perl and JAVA languages, and using the common gateway interface(CGI).