In this paper, we address the problem of formal security verification of transport protocols for wireless sensor networks (WSN) that perform cryptographic operations. Analyzing this class of protocols is a difficult task because they typically consist of complex behavioral characteristics, such as launching timers, performing probabilistic behavior, and cryptographic operations. Some of the recently published WSN transport protocols are DTSN, which does not include cryptographic security mechanism, and two of its secured versions, SDTP and STWSN1. In our previous work, we formally analyzed the security of Distributed Transport for Sensor Networks (DTSN) and Distributed Transport Protocol for Wireless Sensor Networks (SDTP), and showed that they are vulnerable against packet modification attacks. In another work we proposed a new Secure Transport Protocol for WSNs (STWSN), with the goal of eliminating the vulnerability of DTSN and SDTP, however, its security properties have only been informally argued. In this paper, we apply formal method to analyze the security of STWSN.