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Data collection is a fundamental and important issue in wireless sensor networks. Recent research has shown that using mobile nodes to collect and carry data in WSNs has many advantages over static multi-hop routing. In this paper, we focus on the problem of reducing the number of mobile nodes require by data collection in WSNs. Each mobile node visits the sensor nodes, that assigned to it, every t seconds to pick up the cached data. The data gathered by mobile nodes far away from the base station are transferred to the base station through mobile nodes closer to the base station. We formally prove that the problem of data collection in WSNs with the minimum mobile nodes is NP hard. In order to reduce the number of mobile nodes, We propose two algorithms. One tries to reduce the number of mobile nodes and optimize their travel paths, and the other one carefully schedules the movement of all mobile nodes to make sure that they can cooperate with each other to transfer all gathered data to the base station. Our simulation results show that our approach can notably reduce the number of required mobile nodes as much as 46%.