The void fraction and the frictional pressure drop were measured for slug and annular twophase upward flow in vertical tubes. On the basis of the experimental results and the assumption of an annular flow model, some predictions are presented for the shear stress at the interface of the gas and liquid phases and for the flow state of the liquid film. The shear stress at the interface, which is affected by both flow states of the gas and liquid, may be expressed as follows : τ_i=(ζ/4) (ρ_gu_s²/2) where ρ_g is the density of the gas and u_s is the slip velocity. The value of the coefficient ζ, defined as above, is calculated from the experimental results and correlated with non-dimensional variables. For the liquid film an equation relating the liquid flow rate and the film thickness is derived from the shear stress distribution in the film by assuming that the film consists of two layers, a laminar sublayer and a turbulent layer. By applying the experimental results to this equation, the eddy diffusivity in the turbulent layer and the velocity distribution in the liquid film are calculated.