[en] Nonlinear behavior of the collisionless drift instabilities are studied for the universal and current driven modes by means of electrostatic particle simulations in two-and-a-half dimensions. Realistic mass ratios of the electrons to the ions are used in the simulations, where the guiding-center approximation for the electrons and the exact dynamics for the ions are employed. Several nonlinear effects including the quasilinear diffusion of the particle density, the frequency shift due to the ambipolar field, the mode competition among the unstable waves, and the quasilinear diffusion in the velocity space are found to be the dominant mechanisms for the saturation. The stabilization of the collisionless drift instabilities by the magnetic shear has also been studied