This study analyzes the blocking rate in an optical network with multi-granularity optical cross connect (MG-OXC). The MG-OXC architecture is composed of three kinds of granularity (fiber, waveband, and wavelength) in order to resolve the scalability problem due to the exponentially increasing traffic in traditional OXC. Based on the MG-OXC optical networks, a fundamental issue is to develop the algorithms for the routing and wavelength assignment (RWA) with tunnel allocation. In this paper, we have worked from a slightly different angle. We propose a blocking probability model using Erlang loss formula to estimate the performance of tunnel allocation with a different tunnel length constraint. Finally the analytical results, which have proved to be very close to the simulation results, obtained from various kinds of network topologies are shown.