A method for analytical predictions of the thickness effect on fracture toughness, and of R-curves is presented. It is based on the recent observation that the fracture resistance K_R is proportional to the local fracture strain ε_F,αβ, where α = σ₂/σ₁, and β = σ₃/σ₁ characterize the local stress state. As this stress state varies from the surface, where it is plane stress (α = 0, β = 0.61), to the center, where it approaches plane strain (α = 0.81, β = 0.61) for sufficiently thick specimens, the local fracture resistance varies correspondingly. The model proposed here assumes that the measured fracture resistance represents the average of the local G_R values along the crack front. The calculated thickness effect is in excellent agreement with experimental results. From the same model an analytical expression for R-curves is suggested with the additional consideration of the relation between the plastic zone size as a function of crack growth. The preliminary results of the predicted R-curves are in reasonable agreement with the experimental curves.