In this paper we introduce a new dimensioning model for the optical network by the time-dependent blocking probability. This approach is motivated by [6] where the absorption probability based on transient analysis of continuous time Markov chain (CTMC) is used. Unlike the absorption probability model, the blocking probability has been used to dimensioning network for long time and has lower computation complexity. However, using traditional steady-state probability will cause overprovisioning in most cases. So we introduce a time-dependent blocking probability model which is based on the transient analysis of CTMC. Then we propose a novel closed-form transient analysis methods for a single link and and an arbitrary topology network. Applying this approach, the time-dependent provisioning can satisfy dynamically changing traffic demands and avoid overprovisioning problem in optical networks.