The paper develops a theoretical performance analysis model for the online single-hop traffic grooming algorithm in the mesh topology wavelength division multiplexing (WDM) optical network. This is done by developing a link blocking model for traffic grooming based on the continuous time Markov chain and queueing theory, and by extending the Erlang fixed-point approximation for traffic grooming analysis. The results obtained from the analytic model are shown to match well with the numerical results obtained from simulations.