This study investigates the effects of grain size on the mechanical properties of Zn-Sn alloy coatings. Nanocrystalline (average grain size = 78 ± 18 nm) and ultrafine-grained (average grain size = 423 ± 96 nm) Zn-Sn coatings were electrodeposited on steel substrates from gluconate electrolytes containing organic additives. The microstructure, surface roughness and mechanical properties of electrodeposited coatings were investigated using field emission scanning electron microscopy (FE-SEM), three dimensional (3D) surface profilometry, nano-hardness, nano-scratch and nano-wear tests. The average surface amplitude parameters such as mean surface roughness (Sa) and root mean square roughness (Sq) decreased by at least 80% while, hardness increased from 209 ± 66 MPa to 523 ± 140 MPa, due to grain refinement from ultrafine-grained to nanocrystalline structure. Nano-scratch results indicated that a deeper grove was formed on the surface of ultrafine-grained coatings than nanocrystalline coatings during the sliding process. For both coatings coefficient of friction increased gradually over the entire sliding duration and reached to maximum of 0.24 ± 0.04 and 0.12 ± 0.02 in ultrafine-grained and nanocrystalline coatings respectively. Wear volume of the coatings decreased by 64.5% due to grain refinement from ultrafine-grained to nanocrystalline structure.