The reliability level is one of the most significant indexes to be considered for aircraft’s design process. As an aircraft design institute, it is necessary to grasp and trace its aircraft’s reliability level, which should also meet the authority’s requirement. Thus, the paper here performs a reliability assessment methodology study based on aircraft’s failure information in flight test and service period. By making data-collecting requirements，normalizing data table and specifying failure criterion as well as statistical approach for reliability assessment, a systematic reliability assessment process has been created. Meanwhile, starting from the practical engineering, One-sided confidence lower limit method is used to assess the above data, which would give the final reliability assessment outcome. Based on the above methodology, the failure information collected from one aircraft during its one-year flight test and service period is applied to assess its aircraft-level, system-level and device-level reliability. And the final outcome could meet the engineering requirement well. The assessment methodology based on aircraft’s failure information in flight test and service period proposed achieves a full flow track from failure data acquisition, judgment to reliability assessment for aircraft, which could greatly drive the aircraft’s design refinement and its consistent improvement on reliability level.