Abstract: Based on the observational data of three geomagnetic observatories during 10 to 24 years in China, it has been revealed that lunar phaseeffect on Z amplitude variation on magnetically quiet days exists with cetain regularity. The basic feature is the half-month periodicity. The amplitude of the lunar phase-effect may reach 2-8 in value. It should not be neglected in studying the amplitude variation of Z component,