Computer experiments on plasma instabilities due to anisotropic velocity distributions are made to study nonlinear phenomena as well as the validity limit of linear theory using a sheet plasma model. Three kinds of anisotropic distributions are considered; type‐A, perpendicular (to an applied magnetic field) velocity distribution function having a hump atv⊥ ≠ 0, type‐B, bi‐Maxwellian distribution withT⊥ ≫ T‖, and type‐C, loss‐cone distribution. Results of the experiments show that instability due to type‐A distribution is generally very strong and resembles two‐beam instability. One of the characteristics of this is that the strength of the instability is relatively insensitive to the propagation angle. On the other hand, instability due to type‐B distribution is quite sensitive to the propagation angle. A particular feature of the type‐B instability is that, in addition to a growth that is predicted by linear theory, there exist several strong bursts which play an essential role in accelerating plasma along magnetic field lines. After the bursts, the plasma arrives at a stable state with a much reduced ratio ofT⊥/T‖compared with that at the initial state. Another interesting instability is the loss‐cone instability which is caused by the type‐C distribution, an inherent distribution in a mirror magnetic field. When a disturbance propagates almost perpendicularly to magnetic field, an instability is shown to develop due to a coupling of hot ions with cold electrons. With the development of the instability, ions are accelerated along the magnetic field lines and are gradually lost from the mirror field.