In a deregulated electric market, the system operator (SO) is always responsible for system reliability and security. For this purpose the SO must ensure an adequate amount of operating reserve, to be activated automatically (primary reserve for frequency control and secondary reserve for load- frequency control) or manually (tertiary spinning and non- spinning reserve). After the intervention of load-frequency regulation, the regulating band must be restored in order to guarantee the assigned availability of the system; this is obtained by using the tertiary reserve for balancing operations. The evaluation of the necessary operating reserve in a multi- area system is generally carried out on a deterministic basis, using the so called "first contingency security criterion". As an alternative, this paper focuses on a simulation method, based on a probabilistic approach with using a sequential Monte Carlo technique.