The air-traffic management (ATM) is a very complex system. Increasing air traffic and demands on efficiency of the ATM system requires improvement of the ATM system. One of possible improvements is to optimize sectorization of the airspace to improve selected metrics. Each proposed sectorization has to be evaluated to determine impact of changes to current system. Evaluation usually consists of several approaches to cover all aspects of the change - e.g. expert groups, statistical data, real-time simulations, fast-time simulations, etc. The paper presents fast-time simulation system AgentFly used to evaluate proposed variants of sectorization. The simulation is able to compute and evaluate metrics important for a decision making of selected sectorization. This approach is applied to a sectorization of the airspace of the Czech Republic. Scenarios were simulated using current and predicted future air traffic. The AgentFly gathers data related to aircraft and ATC to compute requested metrics. Simulation outputs were used for evaluation and comparison of proposed sectorizations.