In recent years, the size increase of red soil in South China has been often reported after adding acid treatment. In this study 3 red soils derived from three main parent rocks, carbonate rock, basalt and shale in southwest China were selected and collected in eastern Yunnan Plateau in order to explore the regularity and reasons of this phenomenon. The results showed that particle group of <2 μm decreased obviously after soil samples were treated with HCl, mid and mean particle sizes of the 3 red soils increased obviously, particularly of red soil derived from carbonate rock. The results of elemental analysis and mineral analysis showed that the contents of iron oxide in 3 red soils were higher than 100 g/kg. The main clay minerals in red soil derived from carbonate rock were gibbsite and kaolinite, was kaolinite in red soil derived from basalt, and were gibbsite and montmorillonite in red soil derived from shale. After iron oxide were removed from soils by DCB (citrate-bicarbonate-dithionite), particle sizes of the samples were reduced after acid digestion treatment. According to the zero point of charge ampholytoid theory, it is believed that adding acid increased positive charge on iron and gibbsite colloids (ampholytoid) , intensified the aggregation with negatively charged colloids of clay minerals, thus resulted in grain size increase. This result indicates that the pretreatment of acid digestion may not be suitable for laser diffraction method in analyzing particle sizes of red soils highly weathered in south China.