The majority of published papers about ferrite ring pay attention to application in electromagnetic compatibility (EMC) rectification and impedance model. And there are few research on resonance frequency. In order to make better use of ferrite ring to suppress high frequency noise, differential mode choke is regarded as research object to acquire the relationship between coil turns and resonance frequency. Firstly, inductance and parasitic capacitance are supposed to be the key influence factors of resonance frequency by theoretical analysis. Secondly, resonance frequency, inductance and parasitic capacitance of ferrite ring with different coil turns are measured by impedance analyzer. So the relationship of them is quantified, which leads to a conclusion that the increase of coil turns results in the decrease of resonance frequency. Finally, in the case of the electric car rear view mirror motor, electromagnetic interference suppression effect is analyzed in condition of the use of ferrite ring. The result provides that effective working frequency range of ferrite ring decreases with coil turn increases.