As a result of the sedimentation of dispersed particles in channels and ducts, distribution function with respect to the particle sizes changes. Particle sedimentation is affected by the gravitational forces and the diffusional migrations to channel walls. In this article, particle sedimentation in vertical and horizontal channels is investigated depending on Pe and Red numbers. Futhermore, new expressions for the calculation of sedimentation velocity depending on the diffusional migrations in turbulent flow were developed. Finally, a distribution function resulting from the particle sedimentaton was suggested. The suggested mathematical expressions were compared with the experimental results given in the literature and a very good aggrement was found.