In this paper, we will present a study of the time-dependence effect in alumite perpendicular media at different thicknesses. Important parameters of the time-dependence effect such as magnetic viscosity and activation volume are investigated. Viscosity as a function of applied field (viscosity curve) exhibits a short plateau at a low field and then decreases monotonously with increasing field. After correcting for the demagnetizing field, the shape of the intrinsic viscosity curves changes to the well-known shape of the viscosity curve of in-plane media, i. e. , they have a peak near Hc. The intrinsic viscosity curves obtained from the experiments were fitted to an analytical model by Chantrell et al., from which, we found that the effective switching volumes obtained by fitting are much smaller than the column volumes, indicating that the reversal mechanism is incoherent.