Wireless sensor networks face acute security concerns in applications. To achieve security in sensor networks, it is important to be able to defend against Denial-of-Service (DoS) attack recently considered as an extremely threatening attack. In this paper, we propose a DoS detection method via practical entropy estimation on hierarchical sensor networks reflecting resource constraints of sensors. In order to enhance the accuracy of detection even in the various deployments of attack agents, we deploy hierarchically entropy estimators according to network topology, and a main estimator synthesizes localized computation. This entropy estimator is simplified by only multiplication calculation instead of logarithm, in addition to providing higher estimation precision of entropy compared to the conventional entropy estimation. Our simulation results indicate that this hierarchical defense is a feasible method.