Coverage holes may exist in wireless sensor networks (WSNs) due to presence of obstacles or invalid sensor nodes in the sensing field. Normally, the holes make the data routing failure when the nodes transmit their data back to the sink. In this paper, distributed protocols are developed to identify the boundary nodes surrounding the holes of the sensing filed in WSNs without using any location information. Experimental results demonstrate that our algorithm can precisely and correctly identify the boundary nodes even in sparsely sensors deployed regions. Besides, our algorithm can give better performance in terms of control packet overhead and simulation time as compared to previous work.