This study aims to investigate the undrained shear strength of sand with added materials at different mean diameter ratios D50/d50 (D50 and d50 respectively represent the mean diameter of host sand and added materials) in loose state by performing a series of undrained triaxial tests. For this purpose, a very low density index of sand matrix IDmat equaling to 0.00 was employed and the fines of C500 silt, Illite and Speswhite with distinct mean grain sizes were respectively added into the host HN31 sand at different contents Fc (5%, 10% and 15%). For comparison and further analysis, undrained triaxial tests were also carried out on Sand-Sand mixtures, created by adding the HN31 sand into the host HN31 sand at Fc of 5%, 10%, 14% and 18.7%. Results showed that, for low value D50/d50 (e.g., 1 for Sand-Sand), the undrained shear strength Su increased as Fc increased, which might be attributed to the increase of occlusal friction, and in that case the sliding of host sand became difficult. By contrast, in the case of higher value D50/d50 (e.g., 58.33 for Sand-C500, 159.09 for Sand-Illite and 500 for Sand-Speswhite), Su decreased as Fc increased. This was explained by the fact that, with increase of Fc, the quantity of added materials with small size increased, and in that case the sliding of host sand became easy. A critical value of D50/d50 (14.5) was thus deduced, which theoretically verified the negligible effect of Fc on Su. Furthermore, for sand mixed with fines (e.g., C500 silt, Illite and Speswhite clays), Fc has an insignificant effect on the peak value of Δu, while the increase of D50/d50 facilitated the generation of excess pore water pressure after reaching about Δumax. These findings provided useful information for analyzing the mechanical behaviour of sand with added materials in geotechnical practice.