Settling velocity distribution of suspended solids, which is one of the most important factors for the performance of the rock bed contact-purification channel, was determined for urban canal/river water by a new settling tube method. In this method (the plural settling cylinders method), some cylinders are prepared as settling tubes, and the supernatant sample is taken from each of these cylinders with time, so that relatively large volume of sample facilitates more detailed measurement of particulate indices.Results showed about 90% of suspended particles in the canal water were smaller than 20 μm in size and less than 10 cm·hr-1 in settling velocity, while 70% were under 2 cm·hr-1 and 50%, were under 1 cm·hr-1. Pilot plant experiments in AIT showed aeration promoted the settlability of suspended matter in influent. This settling test also indicated that aeration in the head pit can increase the settling velocity of influent particles due to flocculation of fine particles. These results mean that aeration in the head pit is important for enhancing SS removal and improving effluent quality through the rock bed channel.