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We show that the wireless sensor networks can be synchronized by natural environmental signals, such as temperature, humidity and so on. The proposed synchronization scheme is based on the noise-induced phase synchronization theory; the phases of the periodical limit cycle orbits of nonlinear oscillators synchronize with each other by adding a common noise signal to the oscillators. Based on this theory, we synchronize the clocks of the wireless sensor nodes by tuning them according to the phase of their nonlinear oscillator to which natural environmental signals are added as noise. In this paper, first we analyze cross-correlation of the natural environmental signals measured by ZigBee wireless sensor nodes, which are arranged outdoor at about 20m intervals, and show that the cross- correlation among the signals sensed at different sensor nodes are around 0.8 or higher. According to this result, we analyze possibility of the noise- induced phase synchronization with changing cross- correlation between the additive noises, and clarify that the nonlinear oscillators can be synchronized in the cases that the cross-correlation becomes around 0.8 or higher. Finally, we investigate feasibility of the noise-induced phase synchronization by actual data of natural environmental signals sensed at each sensor node and show it is possible to synchronize wireless sensor nodes without any interactions or communications among them.