In this study an attempt is made for assessment of internal instability of granular soils based on an analysis of the variation of the secant slope of the soil particle size distribution(PSD)curve according to three commonly used semi-empirical criteria: the Kezdi criterion,the Sherard criterion and the Kenney and Lau criterion.A computer code ASISGS is developed to first obtain the secant slope curves of soil PSD curves and then the potential of internal instability of 34 soil specimens are analyzed.Comparison with experimental results indicates that the Kenney and Lau criterion is successful in distinguishing between stable and unstable soil particle distributions for a majority of soil specimens.But the Kezdi and the Sherard criteria are not always reliable and are barely applicable when the finer fraction is less than a specific threshold value,especiall y for internally stable soil specimens which would be deemed as internally unstable.The intersections of the secant slope curves and the limited slope lines demonstrate that the threshold values can be taken as 15%and 12.95%for the Kezdi and the Sherard criteria,respectively.Finally,a new synthetical chart is proposed for assessment of internal instability of granular soils and the results derived are in good agreement with experimental results.