This study was aimed at the study of the flow characteristics over a circular crested weir. The experiments were carried out with 7 types of circular crested weir and a model of ogee weir. All models were placed across a flume in the hydraulic laboratory at the Hydraulic Model Test III Bilding, Hydraulics Division, Research & Development Center, Royal Irrigation Department. The experimentation was carried out to study the relationship between the physical characteristics of circular crested weir such as weir height (P), crest radius (R), downstream weir surface (theta) at varius magnitudes of the flow discharge passing the weir (Q) or the flow depth above the weir crest level (H) which effected teh flow characteristics over the weir such as discharge coefficient (Cd), velocity distribution (V), pressure (p) and horizontal force exerted on the weir (F). In addition, the preliminary guideline for the design of these physical parameters of the circular crested weir was determined. Also, the comparision between the circular crested weir and the ogee crested weir was carried out. From the test result for the range of flow rate from 0.64 to 8.83 l/s. or the range of ratio Hฺ/R from 0.217 to 2.471, it was found that the change of weir physical characteristics effects the change of the flow characteristics as follow. Cd is mainly dependent on Hฺ/R and Hฺ/P. Cd increased when Hฺ/R and Hฺ/P increasd. The velocity distribution did not significantly varied with the weir shape. The pressure on the weir and the horizontal force exerted on the weir found to depend mainly on the sum of the energy design head and the weir hight (Hฺ+P) which is close to the upstream flow depth of the weir. As the study result, it recommended that the preliminary guideline for the design of the physical parameters of the circular crested weir is recommended as fallows theta = 45 ํ. P is specified with Hฺ/R in the range from 0.217 to 1.606 such that the pressure over the weir will be positive. From the comparision between the circular crested weir and the ogee crested weir it was found that the values of Cd for both weir shapes were close to each other. Giving the average value of Cd of circular crested weir equal to 0.563 and the average value of Cd of ogee weir equal to 0.556. In addition, the velocity distribution (V), pressure (p), and horizontal force exerted on weir (F) are similar for both weir shape.