In a flexible bandwidth optical network, the necessary spectrum resources on a given route are sliced off into small slots from the available pool and adaptively allocated to the end-to-end optical path according to the client data rate and the available spectral resources. And efficiently assign spectrum slots would decrease the light path blocking probability and enhance the network performance. In this paper, the authors first defined a measurement parameter called contiguous spectrum segment value as the objective value during the resource allocation procedure and then they developed a novel spectrum resource allocation algorithm called MNACSSs (Minimum Number of Available Contiguous Spectrum Segments) which based on the FF (First Fit) algorithm.