The ERA 2013 flexible pavement design manual is primarily used for structural design of flexible pavements in Ethiopia. The manual uses a catalogue of pavement structures for given traffic load and subgrade soil class. The structures in the catalogue were published from TRL Overseas Road Note 31 developed based on empirical design equations derived from tests on section of roads at specific geographic locations with limited types of materials subjecting the test sections to a known traffic load repetition. The ERA manual doesn’t investigate the pavement materials behavior and sensitivity to the conditions it was designed for. However, excess strains on pavement structures coming from the wheel loads are causing the pavement to fail early before their design life. Accordingly, evaluation of the performance of flexible pavements selected from the ERA design manual catalogue of flexible pavement structures was made to investigate if the pavement structures theoretically could serve their design life or fail prematurely. In connection, annual average daily traffic (AADT) counts recently taken on federal road segments of Ethiopia was obtained and used to judge the distribution of the traffic load classes of which T1 and T3 design traffic load classes were found to be the most frequently occurred at the initial and 20 years forecasted traffic respectively. Then, pavement structures were selected from the catalogue under charts A1, A2, A3 and B1 for the distinguished traffic load classes with varying subgrade soil class(S1 through S6). Then, the selected pavement structures were mechanistically modeled with KENPAVE software considering the layered elastic analysis theory and their performances checked for the common distresses of fatigue cracking and rutting(permanent deformation). Finally, a deterministic Life Cycle Cost Analysis (LCCA) was conducted. Results of the mechanistic model output indicated that the horizontal and vertical strain distribution of the pavement structures varied considerably from chart to chart as well as among each cell of each chart and comparison of their performance when a mechanistic-empirical pavement design method was adopted indicated that only 6.9% and 10.34% of the allowable Number of Load Repetitions to fatigue failure (Nf) and rutting failure (Nr) respectively were within bounds of the design ESAs range showing that remaining pavements analyzed were performing beyond the design ESA (conservative design) or failed to resist respective failure modes. Furthermore, flexible pavements showed a wider range of variation in cost for given traffic load and subgrade soil class in which case flexible pavements with unbound granular base are cheaper than those pavement structures with composite or cement-bound stabilized base. Therefore, the ERA flexible pavement design manual structure catalogue needs to be revised considerate of the pavement structures mechanistic behavior and their performance to common distress types occurring in asphalt pavements. In this regard, a different pavement thickness design chart has been proposed to be used for design of unbound thin surfaced granular pavements. The chart will determine the granular pavement thickness for a known design traffic volume and subgrade soil (CBR). Keywords: Mechanistic Analysis, KENPAVE Computer Software, Behavior of Pavements, Fatigue Cracking, Rutting(Permanent Deformation), Life Cycle Cost Analysis(LCCA, Pavement Design Chart.