Garnet nanoparticles Sm2.8-xBixSr0.2Fe5O12 (0.0 ≤ x ≤ 0.15) were prepared by standard ceramic technique where the pre and final sintering were 950 and 1400 oC respectively for 10 hours with heating/cooling rates of 2 oC/min. X- ray diffraction (XRD) analyses were carried out to study the role of bismuth content on the phase formation as well as on the lattice parameter, density, porosity and crystallite size of these samples. The data revealed that the crystallite size decreased with increasing Bi-content from 102 nm at x = 0.0 to 28 nm at x = 0.15. Scanning electron microscope (SEM) showed that, the porosity of the samples was decreased with increasing Bi-content. The magnetization loop (M-H) was measured at room temperature using the vibrating sample magnetometer (VSM). The magnetic measurements clarified that the Curie temperature, the effective magnetic moment and the saturation magnetization increase with increasing Bi content. The obtained results were interpreted based on the garnet structure, the role of Bi as a thermal catalytic agent and the effect of particle size on the magnetic properties. This work recognized that, Bi substitution on samarium strontium iron garnet revealed to establish new nano material used for different technological fields.