This paper focused on the structure, optic and magnetic properties of niobium substituted Co0.5Mn0.5NbxFe2-xO4 nanoparticles (NPs) that synthesized by hydrothermal method. The synthesis of cubic spinel ferrite structures was confirmed via XRD, SEM along with EDX and elemental mapping, TEM and FT-IR. The crystallite size of the products was found between 17 and 18 nm. The %DR-UV-visible spectrophotometer revealed that the band gap values are 0.37, 0.52, 0.54, and 0.46 eV for each substitution respectively. Room temperature (RT) and 10 K magnetization measurements were performed on spinel Co0.5Mn0.5Fe2-xNbxO4 (0.00 ≤ x ≤ 0.10) NPs by VSM. The specific magnetic parameters such as saturation magnetization (Ms), remanence (Mr), coercivity (Hc), magnetic moment (nB) and squareness ratio (SQR = Mr/Ms) were determined and evaluated. The M(H) curves showed ferrimagnetic nature at RT and 10 K. The Nb3+ ion substitution significantly improves the magnetizations data. An enhancement in the Ms, Mr, and nB values was observed in substituted products compared to pristine one. The highest values were obtained for x = 0.04 product. The obtained magnetic data are primarily derived from the substitution of Fe3+ ions with Nb3+ ions that will strengthen the super-exchange interactions.