Abstract: To improve the effective throughput achieved by opportunistic spectrum access of secondary user in hierarchical networks, a robust opportunistic spectrum access policy based on channel quality is proposed. The proposed policy fully utilizes the transmission opportunities with good channel quality by taking both channel quality information and channel occupation information into account to make transmission decisions, which enables secondary user to access channel in sensed idle and busy states with different channel quality thresholds. With this policy, an optimization problem maximizing the effective throughput of secondary user with interference constraint for protectingprimary user is formulated, and the optimal robust channel quality thresholds are derived considering the existence of inaccurate estimation to the channel propagation property. Simulation results demonstrate the effective throughput performance of secondary user can be significantly improved by implementing the proposed policy.