Manganese doped Nickel ferrites nanoparticles (NPs) have been synthesized by modified co-precipitation method for investigating the effects of adding dopant ions after formation of nucleation centers in the reaction medium. The structure and magnetic properties of the as-synthesized samples have been studied by X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FT-IR) and Vibrating sample magnetometer (VSM). The structural results showed that all the samples have cubic spinel phase with the average 21 nm crystallite size. In addition, variation of the preferred XRD peak from (311) orientation to (440) occurred because of the compressive strain in the structure which was calculated by means of Williamson-Hall method (W-H). Adding dopant ions, after nucleation, caused strain, spin canting and reduction of saturation magnetization.