We investigate the effect of permeant flow on the sedimentation of porous fractal-aggregates in water. Our theoretical analysis gives explicit calculations on the scaling behaviours of settling velocities, taking into account the fractality through a proper permeability for fractal aggregates. The calculated results for the scaling behaviours of settling velocities fit remarkably well with experimental data. The analytic expression for the settling velocity provides a criterion for determining fractal dimensions Df of aggregates from sedimentation experiments.