Magnetic properties of Mn-doped ZnO nanoparticles, synthesized by the co-precipitation method, have been studied. The presence of a single crystalline phase identified as the wurtzite structure was determined by X-ray diffraction measurements in all the samples. Additional Bragg reflections associated to manganese oxides phases have been found in the 10 mol% Mn-doped ZnO sample which are more evident after calcinating at high temperatures. Magnetization measurements reveal a paramagnetic behavior with antiferromagnetic interactions becoming weaker as the Mn concentration is increased. For the 10 mol% Mn-doped ZnO sample the coexistence of a paramagnetic and a ferromagnetic phase has been determined. The latter phase was associated with the presence of Mn3O4 compound which is in agreement with the structural results.