Soil aggregates of two karst soils(terra fusca and rendzina) and one zonal soil(red soil) with different soil organic carbon(SOC) contents were analyzed at 1.5, 2 and 2.5 mm sieving strength using AS200. Results show that the three soils, in order of decreasing mean soil weight diameter(MWD), were rendzina, terra fusca and red soil(p<0.05). With the change of sieving strength, particle size fractions of soil aggregates for terra fusca and rendzina were the same, while for red soil, the fraction changed from the dominance by large aggregates(>8, 8~5 and 5~2 mm) to middle sizes(5~2 and 2~1 mm). Red soil was the most easily destroyed soil, terra fusca was the second, and rendzina was the third, according to the mechanical classification. This implies that the mechanical stability of karst soil aggregates was better than that of red soil, and soil aggregate classification for the karst soils was directly affected by sieving strength. From the point of view of soil aggregates, the stability of the karst soils was stronger.