Intrusion detection in Wireless Sensor Network (WSN) is of practical interest in many applications such as detecting an intruder in a combat zone. The intrusion detection is defined as machinery for a WSN to detect the subsistence of unfortunate, incorrect, or anomalous moving attackers. For this purpose, it is a fundamental issue to differentiate the WSN parameters such as node density and sensing range in terms of a desirable detection probability. In this paper, we consider this issue according to two WSN models: homogeneous and heterogeneous WSN. Furthermore, we derive the detection possibility by considering two sensing models: single-singing detection and multiple-sensing detection. In addition, we converse the network connectivity and broadcast reach ability, which are necessary conditions to make certain the corresponding detection probability in a WSN. Our simulation results validate the analytical values for both homogeneous and heterogeneous WSNs.