Internal erosion is one of the main causes of instabilities within hydraulic earth structures. Four types of internal erosion can be distinguished: concentrated leak erosion, backward erosion, contact erosion and suffusion. This paper relates the suffusion which causes the change of grain size distribution, porosity and hydraulic conductivity in the earth dam, dike. According to [1] an expression of the erosion resistance index as a function of fourteen physical parameters: dry unit weight, blue methylene value, internal friction angle, initial hydraulic conductivity, minimum value of ratio H/F, percentage of finer fraction (based on Kenney and Lau’s criteria), gap ratio includes Gr, d5, d15, d20, d50, d60, d90 (diameters of the 5%, 15%, 20%, 50%, 60%, 90% mass passing, respectively) and P (percentage finer than 0.063mm). Besides, the two relation equations for the gap-graded soils and widely-graded soils are also shown. Based on the classification of suffusion through the erosion resistance index (Iα), the paper assesses the suffusion susceptibility of earth dam of Dien Truong reservoir-Quang Ngai. The results show that the earth dam classification is “Moderately Erodible” at the initial section.