A lot of research directed its concern to the reliability of Wireless Sensor Networks (WSNs) used in various applications, especially in early detection of forest fires to ensure the reliability of warning alarms sent by sensors and reduce the average of false warnings. In this research we have tried to evaluate the reliability of WSN used in early detection of fires in Fir and cedar preserve, mainly. By designing hybrid WSN network, similar to the terrains of the preserve and modeling it using program Opnet14.5. We have studied several scenarios, to allow increasing malfunction of the network resulting from fire break out and spreading: starting in allowance of 0% and comparing its results the results of mathematical equations of reliability according to the same scenarios. In addition, we have calculated the final availability through suggesting a mechanism to improve WSN reliability using the redundancy, i.e add sensitive spare nodes, which replace the damaged ones as the result of fire. The results have proved the remarkable increasing of reliability. Also, it has been predicted of the reliability of the network designed according to reliability of different values of the nodes used by using one of the reliability devices "the Block Diagram".