The in-plane inelastic buckling and strengths of circular steel I-section arches are investigated using a finite-element program for the nonlinear inelastic analysis. The elastic-plastic-strain-hardening behavior of the arches is determined by taking into account the effects of the arch curvature, large deformations, high-order deformed curvatures, material inelasticity, initial crookedness, and residual stresses. Radial loads uniformly distributed around the arch axis, concentrated loads, and loads distributed along the horizontal projection of the arch are studied, which induce either uniform compression or combined bending and compression in the arch. The effects of initial crookedness, rise-to-span ratio, residual stresses, and dead load to total load ratio on the in-plane inelastic stability and strengths of steel arches in uniform compression and in combined compression and bending are investigated.