Abstract: The extensional stresses and elongational viscosities of sesbania aqueous solution were measured by a self- made extensional rheometer. The extensional stresses and elongational viscosities gradually increased with strain rate, which explained the significant increase in flow resistance observed in a porous medium. The results also showed that the modified Williams- Bird constitutive equation with four parameters is suitable for either shear flow or elongational flow of sesbania aqueous solution.