Although wireless sensor networks have been studied extensively, several problems should be addressed, including the sensors placement policy, the data aggregation or fusion issue, and realizable applications. However, one of important issues is the placement policy of sensor nodes. Most studies have proposed the probability-based placement policies for monitoring an overall area. In most applications, not entire area is interested to be sensed. Additionally, the fully monitoring of an entire area causes several disadvantages - high cost of deployment, long transmission delay, slow response, and unnecessary data aggregation. Furthermore, previous works lacked of considering the difference between the sensing radius and the transmission radius that might result in inaccurate analysis. Therefore, the authors propose herein an efficient sensor placement approach (ESP) for a sparse interested area with considering of obstructers that block the data transmission among sensors. Meanwhile, the issue of different radiuses of sensing and transmission is analyzed in detail. Numerical results demonstrate that ESP requires the least number of sensor nodes under various network sizes and different number of obstacles. Moreover, simulation results indicate that the number of sensor nodes decreases when the sensing or transmission radius increases. The running time of ESP, O(K<sup>2</sup>), is also analyzed, which is better than that of the probability-based approaches, O(N<sup>2</sup>)> where K is the number of interested grids and N is the number of grids.