Present investigation is conced with the free vibration property of a beam with periodically variable cross-sections. For the special geometry characteristic, the beam was modelled as the combination of long equal-length uniform Euler-Boulli beam segments and short equal-length uniform Timoshenko beam segments altately. By using continuity conditions, the hybrid beam unit (ETE-B) consisting of Euler-Boulli beam, Timoshenko beam and Euler-Boulli beam in sequence was developed. Classical boundary conditions of pinned-pinned, clamped-clamped and clamped-free were considered to obtain the natural frequencies. Numerical examples of the equal-length composite beam with 1, 2 and 3 ETE-B units were presented and compared with the equal-length and equal-cross-section Euler-Boulli beam, respectively. The work demonstrates that natural frequencies of the composite beam are larger than those of the Euler-Boulli beam, which in practice, is the interpretation that the inner-welded plate can strengthen a hollow beam. In this work, comparisons with the finite element calculation were presented to validate the ETE-B model.