Flow behavior of transient mixed electro-osmotic and pressure driven flows (EOF/PDF) through a microannulus is investigated based on a linearized Poisson-Boltzmann equation and Navier-Stokes equation. A semi-analytical solution of EOF velocity distribution as functions of relevant parameters is derived by Laplace transform method. By numerical computations of inverse Laplace transform, the effects of inner to outer wall zeta potential β, the normalized pressure gradient Ω and the inner to outer radius ratio α on transient EOF velocity are presented.