Seven laterite samples derived from Akure, Ondo, Owo, Oshogbo, Ilesha, Ado-Ekiti and Ijan Ekiti in South-west Nigeria were classified using Unified an AASHTO soil classification systems. Unified soil classification was carried out in accordance with ASTM D2487-93 while AASHTO soil classification was carried out in accordance with AASHTO M145-88. The results of the two soil classification systems were correlated. Base on Unified soil classification, Loc.1 to Loc.3 are SM (silty sands), Loc.4 is CH (clayey soil of high plasticity), Loc.5 is CI (clay soil of medium plasticity), while Loc.6 and Loc.7 are SC (clayey sands). Base on AASHTO soil classification, Loc.1 is A-4(0)(silty soil), Loc.2 is A-6(3) (clayey soil), Loc.3 is A-1-b(3) (gravel and sand), Loc.4 is A-7(20) (Clayey soil), Loc.5 is A-7(11) (Clayey soil), Loc.6 is A-7(8) (Clayey soil) and Loc.7 is A-7(6) (Clayey soil). Base on the results of two classification systems, only the results of Loc.2 soil results contradicts each other, hence it can be concluded that Unified AASHTO and soil classification systems give almost the same result and they can be used interchangeable.
Keywords: Unified, Soil Classification, Laterite, Weathering, plasticity, Temperature, Precambrian, In-Situ and undifferential.