Nowadays, statistics show an increase in the diseases of the circulatory system in United Arab Emirates, which makes it the first leading cause of death. This prompted a high demand for adopting an emerging technology of e- Health monitoring systems using Wireless Body Area Network (WBAN), which collects patient's data and communicates it effectively to the relevant healthcare providers for continues monitoring of patient's health conditions. Since patient's data is private, securing the WBAN becomes very essential. In this paper, we propose and implement an energy efficient and lightweight encryption algorithm to secure the communication between a WBAN sensor and a mobile device in e-Health monitoring system. More specifically, our proposed solution adds a layer of encryption to the three-tier architecture used by the WABN to encrypt the patient's vital signs (pulse rate, temperature, respiration rate, and blood pressure) captured by the sensor and sent to a mobile device that a patient carries. Our implementation results show that our Lightweight Encryption Algorithm (LEA) is the more suitable for WBAN environment, where the devices used (sensors and mobile devices) have limited memory space and small processing power are very small, than those of conventional encryption algorithms.