Geometric shape function, f(a/W), is a correction factor that accounts for finite specimen size in calculation of stress intensity factor (SIF). Pin-loading tension method is suitable for fracture toughness testing of axially cracked thin-walled tubular specimens. But unique expression for f(a/W) of such specimens does not exist as it is sensitive to tube geometry. In this work, a generalized form of geometric shape function has been derived through a detailed finite element analysis and the outcome is validated with actual experimental results. Using the general shape function, SIF for any axially cracked thin-walled tube having dimensions within the range can be predicted.