Mobile sites are mobile network access infrastructures that provide cellular mobile network services to network customers in the form of data, voice and internet. Mobile sites fault in Ethiotelecom are more common and challenge operators to realize maintenance cost, maintenance time, revenue loss, number of faults, quality of service and it will reflect big impact to customers. Generally in Ethiotelecom, mobile site fault occur due to transmission link failure, power failure, antenna failure and radio access network failure, etc. To resolve these problem and to increase network availability of mobile network, failure cause and reliability analysis technique in a telecom maintenance strategy is important. In Ethiotelecom, fault monitoring techniques like U2000 and travel ticket monitoring approach are used. U2000 a network management system to monitor and troubleshoot BTS sites down and uptime, while Trouble Tickets (TT) document and track issues reported at these sites. However, these are not sufficient to identify and minimize the fault maintenance time, maintenance cost, frequency of failure, and also not sufficient to increase network availability rate and quality of service. Performing mobile site component level failure cause and reliability analysis is important to make mobile sites reliable and available, to manage faults in the entire network, to increase company performance and to device component spare part forecasting. The study also helps to know the failure behavior of mobile site to ensure safe operation and efficient service flow. In this research, a sample failure data was extracted from the live network in Ethiotelecom within a period of six month, November 2019 to May 2020 from seventeen Debre Markos city mobile sites by using U2000 software, travel ticket data and Operation and maintenance performance-enhancing system (OMPES) tools data. Using these data and PTC Windchill Quality Solution 11.0 Tryout software, failure cause and reliability analysis of mobile site evaluated using the probability distribution method. Using the reliability block diagram analysis techniques, we have found that the reliability of Debre Markos City fiber access mobile sites and microwave access mobile sites are 95.64% and 92.68% respectively. Keywords:- Weibull probability distribution, Reliability, Failure rate, Operational availability.