The main geomagnetic field, B, and its secular variation,Ḃ, originate from the same dynamo process in the outer core. However, their spatial structure and temporal behaviors are drastically different. Using 'globally averaged unsigned annual rate',Ẋ,Ẏ ,Ż,Ḣ andḞ , to express the magnitude of theḂ-field, we studied periodicity of theḂ-field on the basis of IGRF-9 models. The results show that theḂ-field experienced a three-episode variation during the centennial period from 1900 to 2000. The maximum annual rates occurred respectively around 1910∼1920, 1940∼1950, and 1970∼1980, showing a 30-year period. In addition, the rising phase in each episode is much shorter than the declining phase. The governing factor of this periodic variation is proved to be the non-dipole field (mainly the quatrupole field), instead of the dipole field, although the dipole field is dominative over all other multipoles in the main field B.