A homogenization method for beam/bar structure is derived and presented in this paper. The beam/bar structure is assumed to be made of repeating unit cells, whose sizes are much smaller than the length of the structure. The macroscopic deformation of beam/bar structure is described based on the Euler-Bernoulli beam theory and on the torsion of circular shaft. Perturbed displacements are computed by three dimensional finite element method for unit cell, representing a unit of periodic structure. Procedures to calculate the perturbed displacement and an expression for effective stiffness of structure are presented. A few numerical examples are presented to demonstrate the validity of present method.