An orthorhombic polycrystal is an orthorhombic aggregate of tiny crystallites. In this paper, we study the effect of the crystalline mean shape on the constitutive relation of the orthorhombic polycrystal. The crystalline mean shape and the crystalline orientation arrangement are described by the crystalline shape function (CSF) and the orientation distribution function (ODF), respectively. The CSF and the ODF are expanded as an infinite series in terms of the Wigner D-functions. The expanded coefficients of the CSF and the ODF are called the shape coefficients s l m0 and the texture coefficients c l mn , respectively. Assuming that C eff in the constitutive relation depends on the shape coefficients s l m0 and the texture coefficients c l mn , by the principle of material frame-indifference we derive an analytical expression for C eff up to terms linear in s l m0 and c l mn , and the expression would be applicable to the polycrystal whose texture is weak and whose crystalline mean shape has weak anisotropy. C eff contains six unspecified material constants (λ, µ, c, s 1 , s 2 , s 3 ), five shape coefficients (s 2 00 , s 2 20 , s 4 00 , s 4 20 , s 4 40 ), and three texture coefficients (c 4 00 , c 4 20 , c 4 40 ). The results based on the perturbation approach are used to determine the five material constants approximately. We also find that the shape coefficients s 2 m0 and s 4 m0 are all zero if the crystalline mean shape is a cuboid. Some examples are given to compare our computational results.