Wireless Sensor Networks (WSNs) is a prevailing technology in assistive environments. Assistive environments may include both home and work spaces such as factories, military installations, industrial spaces, and offices. Critical quality-of-service properties of WSN are reliability, availability, and serviceability. This paper focuses on reliability for healthcare applications. Reliable WSN-based monitoring services can prevent accidents, improve the quality of life, and even help with early health diagnosis and treatments. However, because patients/the elderly may have cognitive or other health problems, the reliability is the dominant factor of quality of services of WSN. This paper presents an approach to analyze the reliability of a WSN with the most popular tree structures. The analysis is based on two distribution models, exponential distribution and Weibull distribution. The simulation results also give options to users on the cost vs. reliability issue.