This paper proposes a method to approximate the out-of-plane effective bucklimg length for web members of circular tubular trusses with unstiffened branch joints. Using a nonlinear buckling analysis, the effective length is determined for a number of tubular trusses with various dimensions. Based on the numerical results, the effective length is represented by a simple formula. Comparisons with the numerical results proves the very high accuracy of the proposed formula for buckling loads.