137Cs activities in purple surface soil on grass land have a good relationship with the medium grain size (d50) by sieving method,but do not have a defined relationship with the medium grain size (d50) by normal method. 137Cs is mainly absorbed to surfaces of soil particles, and 137Cs concent rations of the mineral and rock particles in surface layers of the aggregates should be higher than those of the particles in the inner layers of the aggregates. As organic concent rations of the samples of medium size are higher than those of both coarse and fine samples, there is no relationship between 137Cs activities and organic concent rations.