The　reinforced　concrete　(RC)　circular　element　is　usually　simplified　as　slab　one　on　the　issue　of　chloride　diffusion　simulation,　without　considering　the　effect　of　the　geometrical　shape.　In　the　paper,　a　modified　slab　diffusion　model　is　proposed　for　circular　section.　A　formulation　for　estimating　the　error　caused　by　neglecting　the　effect　of　shape　on　chloride　diffusion　is　derived.　The　formulation　demonstrates　that　radius　significantly　affect　the　error.　When　shape　is　neglected,　the　effects　of　model　parameters,　including　the　diffusion　coefficient,　radius,　cover　concrete　thickness　and　age　factor,　on　the　corrosion　initiation　time　are　investigated.　The　result　shows　the　radius　has　a　slight　effect　on　calculating　the　corrosion　initiation　time　compared　with　other　model　parameters.　Furthermore,　the　influence　of　shape　on　estimating　on　reliability　index　for　different　service　time　is　also　discussed.　A　guideline　is　proposed　for　properly　using　the　modified　slab　diffusion　model　instead　of　the　original　one　to　predict　service　life.　Finally,　the　impact　of　the　shape　of　the　RC　circular　column　on　the　durability　design　against　chloride　corrosion　is　studied.　The　design　result　when　the　column　is　simplified　as　a　slab　element　indicates　a　lower　required　minimum　concrete　cover　thickness.　The　minimum　thickness　should　be　improved　by　5　mm　as　a　conservative　choice　based　on　the　result　of　the　slab　element.