Cylindrical weir is one of the hydraulic structures used in open channels and reservoirs to regulate and control the water levels. Compared to the other shapes of structures, cylindrical weir has some advantages such as easy flowing of floating objects over it, simplicity of its design and construction, less loss, and greater discharge coefficient. Much work has been long done on the hydraulic of cylindrical weirs in non-roughness conditions. However, the effect of the roughness of the structure body that can affect discharge coefficient, flow loss, and formation of the critical depth as well as the characteristics of the boundary layer has received little attention and lack of comprehensive research on this is quite evident in previous studies. Therefore, in this study, the effect of roughness on the hydraulic of cylindrical weirs has been analyzed in details with different diameter, discharge and roughness in the laboratory and finally the obtained results was compared to other researchers. Experimental model is composed of full cylinder with three different diameters and four levels of roughness of PVC, 1.5, 3 and 4.5 mm for each diameter. The results shows that for the same cylinder and with the equal flow rate, an increase in the roughness of surface  of the weir from PVC to 4.5 mm, results in a reduction of cd value around 10.7% and leads to an increase of  energy loss about 22.3%. Moreover, it has been concluded that the critical depth formed about 78.8% closer to the crest. While the effect of roughness on the discharge coefficient in smaller diameters is more visible, the effect of roughness on energy loss doesn’t depend on the diameter of cylinder. Furthermore, there is a decreasing in cd and energy loss value with increasing of the cylinder diameter under the same surface roughness. By increasing of the inlet discharge, discharge coefficient increases and energy loss decreases and the distance of critical formation point from crest, increases. The achieved results regarding the dimensionless ratio of crest depth on the critical depth (H_crest/H_c) is in conformity with the results achieved by chanson and montes`s research in the case of non-roughness.