Cerium oxides are prevalent catalytic materials, and the lanthanide-doped ceria have attracted special interest since it is easy to tune the concentration of oxygen vacancies (V-O) by changing the doping content. The presence of V-O is generally believed to favor a catalytic reaction, but the formation of dopant-vacancy associations at a high doping concentration might produce an adverse effect. Herein, evolutions of the structural properties and catalytic performances in Sm-doped ceria (SmxCe1-xO2-delta, x = 0-1) are investigated to explore the doping effect of Sm3+ on the ceria-based nanoctrystals. The SmxCe1-xO2-delta films composed of nanoctrystals are elaborately prepared via electrodeposition under mild conditions to prevent phase separation. A combination of studies, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman, photoluminescence (PL), and methanol electro-oxidation (MEO) reaction, reveals that variation trends for the V-O concentration and catalytic property of SmxCe1-xO2-delta are unsynchronized. The lattice structures of SmxCe1-xO2-delta nanoctrystals undergo a smooth and steady transition from F-type (fluorite CeO2) to C-type (cubic Sm2O3) with the increase of Sm3+ contents. The structural transition occurs in the Sm3+ concentration range of 64-84%, within which the V-O concentration reaches the maximum as well. However, the optimal MEO performance is obtained at a relatively lower doping concentration of 24%. Above this concentration, significant dopant-vacancy associates are observed by XRD, Raman, and PL characterizations. It is inferred that, for these ceria-based nanocrystals, the dopant-vacancy association induced by high doping would impede the growth of catalytic performance despite all the benefits of V-O.