Abstract: The out-of-Plane stability of compressive truss diagonals is a spatial problem which has not been well studied. The Chinese GBJ17-88 Specification of Steel Structure Design Prescribes that the effective length of these members shall be equal to their geometric length between panel points. In case one of the ends (or both ends) is not fully supported laterally, other members connected to the end have to provide an elastic support of adequate stiffness in order that the diagonal could be designed as hinged-hinged compression member. Otherwise, the effective length of the diagonal should be larger than its geometric one.In this paper, the stiffness provided by either compression or tension chords in different cases is analysed, and the effect of initial crookedness of compression chord is taken into account.Formulas are Presented to compute the required stiffness of chord member and the effective length factor for inadequately constrained compression diagonals. The decrease in flexural rigidity of chord members of monosymmetric section due to torsion is also discussed.