In the Internet of things (IoT) applications where things are connected and exchanged data wirelessly, security places an important role. In addition, IoT sensors have limitation on the power source and computing ability. Therefore, developing a wireless communication transceiver which is high-security, low power consumption, and low complexity for IoT sensors is significant. With the advantages such as long range, low power, and low-cost, IEEE 802.11ah is a promising candidate for developing IoT sensor's wireless transceiver. However, the current version of 802.11ah standard does not focus on enhancing the system security. In this research, we solve that problem by proposing a low complexity and high-security encryption method at the physical (PHY) layer of 802.11ah. Because the encryption is processing at the PHY layer, we built the simulation to evaluate the effect of the encryption method to bit error rate (BER) and packet error rate (PER) performance of the system. Our research shows that the implementation of our encryption does not affect to the BER and PER performance of the system while some conventional methods do. In addition, the simulation also shows that unexpected users are entirely unable to obtain the transmitted information without the correct key.