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Vehicles with automated manual transmissions (AMT) for gear shift control offer many advantages in terms of reduction of fuel consumption and improvement of driving comfort and shifting quality. Recently, AMT are proposed to be connected with epicyclic gears and friction brakes, in order to reduce the torque gap during the up-shifts. This paper is focused on a dynamic nonlinear model of epicyclic gears with two degrees of freedom. It allows to take into account load dependent power losses, gearbox bearing losses and friction between gears; moreover, inertia effects of gears are introduced in the model. The proposed approach is implemented in a driveline model of an automated manual transmissions (AMT) for gear shift control, in order to evaluate transient and steady-state behavior of epicyclic gears. Efficiency expressions are finally obtained under the assumption that the energy losses of the epicyclic gear are equal to those of the corresponding ordinary gear.