Rotational instabilities in rigidly rotating field‐reversed configuration (FRC) equilibria are studied using a quasineutral hybrid simulation code. We observe unstablem=2 modes at levels of ion rotation below instability thresholds predicted by finite‐Larmor‐radius fluid theory. Nonlinear effects are found to reduce the growth rate and lower the real frequency of them=2 mode at large amplitude. Instabilities withm>2 have been observed for strongly reversed cases. It is also found that growth rates for these instabilities can be reduced by increasing the ratio of the plasma radius to the ion Larmor radius.