The main geomagnetic field, B, and its secular variation, , 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', , , , and , to express the magnitude of the -field, we studied periodicity of the -field on the basis of IGRF-9 models. The results show that the -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.