A pure ferrite and epoxy samples as well as the epoxy/ferrite composites with different 20 wt.%, 30 wt.%, 40 wt.%, and 50 wt.% weight ferrite contents have been prepared by the chemical co-precipitation method. AC-conductivity and dielectric properties such as the dielectric constant and dielectric loss of the prepared samples have been studied. The obtained results showed that the samples had a semiconductor behavior. The dielectric constant of the composites has been calculated theoretically using several models. For the composite sample that contains 20 wt.% of ferrites, these models give satisfactory compliance, while for the composite samples with a higher percentage of nanofillers, more than 30 wt.% theoretical results do not coincide with experimental data. The investigated polymer has very low conductivity, so this type of polymer can be useful for high-frequency applications, which can reduce the losses caused by eddy current. Thus, the prepared samples are promising materials for practical use as elements of microwave devices.Nanomaterials 2020, 10, 492 2 of 16 Nanotechnology can be an excellent tool in the study, preparation, research, and use of new perspective materials, as well as in solving the problem of particles sizes in the range of 1-100 nm, where unusual phenomena allow new applications [2][3][4]. Tiny nanoparticles with diameters of some nanometers usually look like molecules. Therefore, the atomic and electronic structures of such small nanoparticles possess different features, certainly not the same as those of the bulk materials. The size-dependent abilities of nanoparticles include electronic, magnetic, optical, and chemical substance characteristics. Nanoparticles can be crystalline or amorphous [5].Nanosized ferrites could be prepared by many methods, including mechanized milling, ultrasonic cavitation method, radio rate of recurrence plasma, hydrothermal technique, reverse micelle method, sol-gel technique, citrate precursor, and co-precipitation technique [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. It is advisable to possess a synthesis method that allows for governor on the nanoparticle dimension and produces nanoparticles having a significant size distribution [2]. The chemical substance co-precipitation method is the most excellent ideal for the formation of nanoparticles due to its ease and much-improved handle over the crystallite dimension and other qualities of the components [22]. Also, lately, magnetic composites comprising magnetic materials alongside polymers comprise a new era of multifunctional components that mix the properties from the constituents; these components are known as magneto-polymeric components [23].The characteristics of ferrites are highly susceptible to methodical handling, sintering conditions, and impurity. Nickel-zinc ferrite is one of the first widely used ferrite produced for an open range of applications that recognizes to high saturation magnetization, high resistance, high Curie temperature, high chemical stability, low coerciv...