  We theoretically investigate four types of dynamical instability, in
particular the periodic and oscillatory type $I_O$, in an anti-ferromagnetic
spin-1 Bose-Einstein condensate in a nonzero magnetic field, by employing the
coupled-mode theory and numerical method. This is in sharp contrast to the
dynamical stability of the same system in zero field. Remarkably, a pattern
transition from a periodic dynamical instability $I_O$ to a uniform one $III_O$
occurs at a critical magnetic field. All the four types of dynamical
instability and the pattern transition are ready to be detected in $^{23}$Na
condensates within the availability of the current experimental techniques.
