Wireless sensor networks are increasingly being used in applications where QoS and low cost are the overriding considerations. Conventional schemes of using sensor nodes and incorporating these three phenomenons (reliability, availability and serviceability) to attain QoS can effectively improve not only the reliability of the overall WSNs but security as well. We discuss the reverse look of QoS and hence present mathematically the three significant quality factors that should be currently taken into account in developing wireless sensor networks application services and security. We also discussed specific characteristics and constraints of wireless sensor networks, QoS factors required when developing security applications for such networks. To evaluated possibility of establishing secure wireless sensor network through QoS we have used Hawk nodes to demonstrate how security can be archived through QoS experimentally. The flow models show how the QoS can be integrated to increase the security and applications running in WSNs.