<b>Effects of Sub Grade and Sub Base Materials Quality on Flexible Pavement A Case Study</b> Flexible pavements are composed of asphaltic layer, base course sub base materials which laid over a well compacted and strong sub grade foundation soil. Basic engineering properties of Sub grade materials are required for design and Coarse grained soils serves as good for supporting pavements whereas fine grained soils, particularly clayey soils pose problem to pavement. Jimma town is a big trade center and coffee production areas of in Ethiopia but, the current existing condition most parts of the road are deteriorated. The objective of this research was to investigate effects of subgrade and sub base materials quality on flexible pavement. The distress is categorized by level of severity according to Ethiopian Roads Authority ERA standard specification to get 4 stations high deterioration of block crack, 6 stations medium deterioration of Alligator crack and 5 stations low deterioration of edge crack of the studied stretch along Ajip to Gebrel church of 10 km in Jimma town. The samples taken randomly from the deteriorated area of the subgrade and sub base materials of 15 stations and laboratory tests conducted and also the results were discussed, compared with the ERA, AASHTO and ASTM. The tests result NMC OMC indicated that excess water in the subgrade and sub base materials according to AASHTO D 180 specification, compaction of subgrade and sub base layers are below the minimum requirement MDD 97 laboratory density of AASHTO, poor strength CBR 1.76gm cc of AASHTO T 193 and ERA specification, sieve analysis for gradation out of the upper and lower limit of AASHTO T 27 for the subgrade and sub base materials. These can be causes of pavement deterioration on flexible pavement. But, the Atterberg’s limit test results satisfy the standard specification of ASTM D 4318. From this research recommended that the designer and contractor should be follow the minimum requirement set by standard specification regarding the engineering properties of materials, the side ditch must be constructed so as to prevent infiltration of water to the under lining strata, seal coats shall be applied to prevent infiltration of water through cracked surfaces to subgrade layer of the pavement. Deterioration, Distress, Moisture Content, Pavement, Sub base and Subgrade 835-845 Issue-6 Volume-2 Getu Tamiru | Palani Ponnurangam