In this paper. we consider an admission control algorithm for cellular networks. It is based on direct/dynamic monitoring of the QoS performances - the delay tail of the admitted users. The main purpose of the direct monitoring of QoS performances is to guarantee the required QoS provision more precisely. The system delay tail is effectively computed by the proposed delay tail estimation algorithm. With the estimated delay tails, each base station determines the admission or rejection for a new user to the system. By doing so, the admission control algorithm becomes more robust against time-varying fading channel environment. Simulation results in OFDMA cellular system shows that the proposed algorithm can provide substantial capacity gains in offering QoS. We also use a general Neyman- Pearson-like framework in evaluating various admission control mechanisms.