Abstract: Dynamic tests on three simply supported beams with external prestressing were carried out. The expermental results indicated that natural frequencies of species increased with the growth of prestressing at the stage of stretching, which is contrary to what derived from classical linear-plastic theories. Also, natural frequencies of species decreased significantly after crack occurs. Natural frequencies of species were calculated with existing formulas and compared with test results, and significant difference was observed between the values of natural frequencies, as well as their varying tendencies. Based on test results and considering concrete as a composite material, a new calculation method of natural frequencies is presented, which generates consistant results with testing observations.