This paper deals with the roughness effects ( ΔC_F ) on skin friction from a hydrodynamical point of view. The former part of the paper consisted mainly of the experimental reports, that is, the devices of pipe flow experiments, the friction measurements in the roughened pipes and the analyses of their data. The main results obtained are as follows; (a) The measured frictional resistances of variously roughened pipes are shown in Fig. 5. In the figure, the resistance curves of painted pipes show clearly different tendencies from those of sand roughened pipes as mentioned by Prof. Sasajima in 1954. (b) The hydrodynamical relation between the roughness height, measured by BSRA roughness analyser, and the so called equivalent sand roughness height, determined by comparing Nikuradse's experimental results, is shown in Fig. 6. The relation can be expressed also by K_s = 1.47 K _BSRA The Latter part of the paper consisted mainly of the theoretical considerations of C_F and their applications. The main results obtained are as follows; (c) Using a logarithmic type of velocity distribution in the boundary layer, the local ΔC_f can be expressed simply by local ΔC _{f 0} and roughness function ΔU / u * as ΔC_f ∝ C _{f 0}^3/2 ( ΔU / u *). (d) The procedures to determine roughness function ΔU / u * from the measured resistance data are explained. It can be done without measuring velocity distribution in the boundary layer. (e) A new roughness parameter h ≡ kF_n ^2/3 ( g /ν²) ^1/3 is proposed, concerning the similarity law for ΔC_F . As the parameter can be applied commonly to both the model and the ship, the new parameter may be a powerful tool for estimating ΔC_F . (f) As an application of the above stated considerations, the frictional resistances of a tanker are calculated in the various roughness conditions. The results are shown in Fig. 12.