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This paper proposes a cooperative jamming strategy for two-hop relay networks where the eavesdropper can wiretap the relay channels in both hops. The problems of jamming beamformer design and power allocation are investigated jointly for two scenarios where the eavesdropper has either a single or multiple antennas, with the assumption that the global channel state information (CSI) is available. Under a constraint that the jamming signal lies in a subspaces orthogonal to the channels to legitimate nodes, we derive closed-form solutions for the jamming beamformers. Based on these results, we find the optimal solution for power allocation via geometric programming.