Multicasting in the optical layer has gained significant importance in the recent years due to several factors. Most of the research work in this area concentrate either on minimizing the number of wavelengths required to meet a given static demand or on multicast route selection algorithms to achieve efficient utilization of fiber bandwidth. Very few significant research has been found, to the best of authors' knowledge, in developing an analytical model for evaluating the blocking performance of tree establishment in optical networks, which motivates this research. In this paper, an analytical model for evaluating the blocking performance of multicast tree establishment in time-space switched optical networks is developed. The performance of different switch architectures are then studied using the analytical model. it is observed that if the multicast tree has very low degree of branching, the blocking probability of establishing the tree is the same as that of establishing a path with same number of links.