<p>The paper presents results of a laboratory investigation into the effect of non-plastic fines on the monotonic behavior of sand-silt mixtures. For this purpose, drained and undrained triaxial monotonic tests were performed on two clean sands and their mixtures with a non-plastic silt. The two different granular soil mixtures have different values of the ratio of fine grains mean diameter, d₅₀, and coarse grains mean diameter, D₅₀. The Critical State theory known as a characteristic state of soil behavior and the equivalent state concept were used to interpret the laboratory tests results. By estimating the equivalent state concept parameters b, and m, the equivalent intergranular, (e_g)_eq, and interfine, (e_f)_eq, void ratios are calculated to determine a single Critical State Line, CSL, in the (e_g)_eq−log(p′) and (e_f)_eq−log(p′) planes, respectively, independently of fines content, f_c. It is shown that parameters b and m depend on f_c, the grains’ shape and the ratios d₅₀/D₅₀ and D₁₀/d₅₀, while (e_g)_eq and (e_f)_eq prove to be suitable parameters for the estimation of the monotonic behavior and undrained critical state strength of granular mixtures, up to their threshold fines content, f_cth, and for f_c &gt; f_cth, respectively, through a unique CSL independently of their fines content. The effectiveness of state parameter, ψ, and equivalent state parameters, (ψ_g)_eq, and (ψ_f)_eq, in the estimation of the undrained critical state strength of sand-silt mixtures is confirmed. Diagrams are proposed, for the estimation of b and m, based on this study’s results and the reanalysis of laboratory datasets of other researchers.</p>