Grain size distributions of sediments of paved (PFR) and unpaved (UPFR) forest roads were compared to an undisturbed forest land (UA) in Belgrad forest located in Istanbul, Turkey. Sediments were collected from sediment traps on the road ditches. Monthly sediment production on UPFR was significantly higher than those of PFR and UA. Sediments were sorted into grain size fractions of >76.20, 50.80, 38.10, 25.40, 19.10, 9.52, 4.76, 2.00, 0.42 and 0.074 mm. Mean rate of passing amount (%) and grain size fractions of sediments were calculated and evaluated by ASTM D422-63 and USCS (Unfined Soil Classification System) classification systems. Significantly more sediment amount passed in all sieve sizes of sediments of UPFR than those of PFR and UA. Furthermore, our results demonstrated that UPFR produced higher rates of erosion, and higher concentrations of all sediment grain sizes than PFR and UA.