Nanocrystalline ZnO particles doped with different compositions of transition metals (TM, i.e., Fe, Ni and Mn) were successfully synthesized at low temperature (80 °C) by a co-precipitation method. The samples were characterized using a vibrating sample magnetometer, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, UV-Vis spectrophotometry and electron spin resonance (ESR). The results demonstrated that the doping concentration significantly affects the structural and magnetic properties of the samples. Room temperature ferromagnetic order was observed in all of the samples and was strongly dependent on the TM dopant. The correlation between the structural and magnetic properties as a function of the TM dopant is discussed.