Data storage technologies have been recognized as one of the major dimensions of information management along with the network infrastructure and applications. The prosperity of cloud computing requires the migration from server-attached storage to network-based distributed storage. Along with variant advantages, distributed storage also poses new challenges in creating a secure and reliable data storage and access facility. The data security in cloud is one of the challenges to be addressed before the novel pay-as-you-go business model can be accepted and applied widely. Concerns are raised from both insecure/unreliable service providers and potential malicious users. In this article, we analyze the integrity vulnerability existing in the current cloud storage platforms and show the problem of repudiation. A novel non-repudiation (NR) protocol specifically designed in the context of cloud computing environment is proposed. We have also discussed the robustness of the NR protocol against typical attacks in the network environments.