The aim of this paper is to characterize the nonnegative functions ϕ defined on (0, ∞) for which the Hausdorff operatoris bounded on the Hardy spaces of the upper half-plane H p a (C + ), p ∈ [1, ∞]. The corresponding operator norms and their applications are also given.2010 Mathematics Subject Classification. 47B38 (42B30, 46E15).