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In wireless sensor networks (WSNs), it is difficult to achieve a large data collection rate because sensors usually have limited energy and communication resources. Such an issue is becoming increasingly more challenging with the emerging of information-intensive applications that require high data collection rate. To address this issue, in this paper, we investigate the throughput capacity of WSNs, where multiple mobile relays are deployed to collect data from static sensors and forward them to a static sink. To facilitate the discussion, we propose a new mobile-relay-assisted data collection (MRADC) model. Based on this model, we analyze the achievable throughput capacity of large-scale WSNs using a constructive approach, which can achieve a certain throughput by choosing appropriate mobility parameters. Our analysis illustrates that, if the number of relays is less than a threshold, then the throughput capacity can be linearly increased with more relays. On the other hand, if the number is greater than the threshold, then the throughput capacity becomes a constant, and the capacity gain over a static WSN depends on two factors: 1) the transmission range and 2) the impact of interference. To verify our analysis, we conduct extensive simulation experiments, which validate the selection of mobility parameters and demonstrate the same throughput behaviors obtained by analysis.