Abstract: Gradation test is used to get the dry mass percentage of various sizes of soil particles. This is the typical test to obtain the actual soil particle size distribution curve. The curve is usually utilized as a preliminary approach to estimate the soil engineering property or used in determining the applicability of backfill materials. A significant component of the test is hydrometer analysis which is used to get distribution for the soil particle with size less than 0. 075mm. However,hydrometer test result is usually different even for the same sample by different operators or laboratories. The hydrometer is the most direct approach to obtain the percentage of silt and clay particles although its result is very sensitive to the temperature or its accuracy depends on various factors. This paper is to investage the background of the errors from the stand point of testing and to summarize the misunderstandings of temperature correction method in the current code. A KONGBAI approach is finally recommended in eliminating or reducing the hydrometer analysis errors. This method can be used as a reference in engineering and can be testified by future lab testing.