Undoped and Mn-doped ZnO nanoparticles (Zn[Formula: see text]MnxO), with nominal weight percentages [Formula: see text], have been synthesized by co-precipitation technique. The synthesized nanoparticles are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV) and Fourier transform infrared spectroscopy (FTIR). From XRD analysis, the compound ZnMnO3 is formed for [Formula: see text] with cubic structure ([Formula: see text][Formula: see text]Å) and its concentration increases with x. Moreover, XRD analysis reveals the wurtzite hexagonal crystal structure for ZnO. The lattice parameters (a and c) of Zn[Formula: see text]MnxO are calculated and they increase with the doping concentration of Mn as a consequence of the larger ionic size of Mn[Formula: see text] ions compared to Zn[Formula: see text] ions. The crystallite size is calculated for all the samples using Debye–Scherrer’s method (SSM), Williamson–Hall methods (UDM, USDM and UDEDM) and Size-Strain Plot method (SSP), and the results are in good agreement with TEM. The presence of functional groups and the chemical bonding is confirmed by FTIR spectra that shows a peak shift between undoped and doped ZnO. The energy bandgap [Formula: see text] is calculated for different concentrations of Mn [Formula: see text] by using the UV-visible optical spectroscopy, between 300[Formula: see text]nm and 800[Formula: see text]nm, showing a noticeable drop in [Formula: see text] with x. At room temperature, the magnetization of the samples reveals the intrinsic ferromagnetic (FM) behavior of undoped ZnO, ferromagnetic behavior of ZnxMn[Formula: see text]O [Formula: see text] and the co-existence of ferromagnetic and paramagnetic behavior for ZnxMn[Formula: see text]O [Formula: see text]. This ferromagnetism is decreased for the doped samples as a consequence of antiferromagnetic coupling between Mn ions. The two samples correspond to [Formula: see text] and [Formula: see text], tend to be superparamagnetic because of the formation of single domain particles as a consequence of small particle size. [Formula: see text] shows an optimum value of Mn concentration for maximum saturation magnetization and the best ferromagnetic nature.