Particle shape represents one of the key parameters that significantly influences the mechanical response, shear strength, deformation, and settlement characteristics of cohesionless soil deposits when subjected to static or dynamic loading conditions. Shear behavior of coarse-grained soils (gravel, gravelly sand, sandy gravel, sand, silty sand, or sandy silt) depends heavily on the two essential particle shape characteristics, which are termed roundness and sphericity. However, there are limited published studies available in the literature that deal solely with the effects of particle shape on shear-strain characteristics and shear behavior of granular soil deposits. Therefore, the veritable role of the particle shape parameter on soil behavior remains incomplete and requires further investigation. The present research work attempts to investigate the effects of particle shape on shear response of different categories of sand-silt mixtures under static triaxial loading conditions. For this purpose, a series of undrained compression triaxial tests was carried out on mixtures of Chlef (Algerian) rounded sand, Fontainebleau (France) subrounded sand, and Hostun (France) subangular sand mixed with low-plastic (Ip=5 %) rounded silty fines. All the sand-silt mixture samples were reconstituted at an initial relative density (Dr=52 %) and subjected to a constant confining pressure (P’c=100 kPa). An evaluation of particle shape characteristics of the tested materials (sand and silt) was performed using a digital microscope device. Two new indexes, termed combined roundness and combined sphericity based on the combination of sand and silt to account for the coupled effects of particle shape and fines content, were proposed. The test results are used to correlate the undrained shear strength of the silty sand soils to the particle shape characteristics. Therefore, particle shape appears to be an important soil index property that needs to be adequately identified, particularly for silty sand–silt mixture soils. The systematic identification of particle shape characteristics leads to a better understanding of silty sand behavior.