Sidewall tapering is often observed during plasma trench etching. In this paper, two types of trench tapering, intrinsic tapering and passivation-induced tapering, are discussed based on numerical simulations and theory of surface evolution. Intrinsic tapering occurs when the etch rate C(theta) decreases rapidly as the slope angle theta approaches that of the vertical surface (i.e., theta = +/- pi/2). It is the dominant mechanism for the formation of tapered sidewalls when the sticking coefficient J is small. For a larger sticking coefficient, passivation-induced tapering becomes more dominant. Quantitative relations between etched trench profiles and some system parameters such as sticking coefficients, etch rates, and re-emission distributions are also presented.