Coagulation is the important step for removal of turbidity in surface water. Effective operation of the coagulation considerably affects sedimentation and filtration in surface water supply system. This research was intended to study the effects of velocity gradient and detention time on the coagulation in polyvinyl chloride pipe and galvanized steel pipe and feasibility study in technic for coagulation in pipe. The parameters under study were velocity gradient(G) and detention time(t). The experiment were conducted on a lab-scale unit which made of polyvinyl chloride pipe and galvanized steel pipe The range of G in polyvinyl chloride pipe varied from 12 to 420 sec.(-1) and t varied from 3.98 to 612.81 sec. The range of G in galvanized stell pipe varied from 26 to 550 sec.(-1) and t varied from 3.71 to 285.71 sec. The experimental results revealed that the efficiency of turbidity removal depended on G and t. Coagulation in polyvinyl chloride pipe, the velocity gradient which gave the maximum efficiency for turbidity removal(G) was 130 sec(-1) at t 102.33 sec. In galvanized steel pipe, the velocity gradient which gave the maximum efficiency for turbidity removal(G) was 340 sec(-1) at t 5.19 sec. Over 80 persent of turbidity was removed at that G and t. Moreover, the results showed that coagulation in pipe could remove turbidity down to the acceptable level for filtration when raw turbid water was not over than 50 NTU. Finally, according to this research, coagulation in polyvinyl chloride pipe had less turbulence than coagulation in galvanized steel pipe because the inner surface of polyvinyl chloride pipe had less roughness than galvanized steel pipe and this effected the less detention time in polyvinyl chloride pipe The detention time of coagulation in polyvinyl chloride pipe were 19.72 over than coagulation in galvanized steel pipe while the price of polyvinyl chloride pipe were 2.74 less than galvanized steel pipe but galvanized steel pipe is easily corroded.