The Raman spectra of MBE-grown GaAs_(1-x)Sb_x in a relatively wide range of composition are reported for the first time. It is observed that the frequency shift of phonons strongly depends on the composition x of Sb. The two-mode behavior of optical phonons is observed only when the composition x of Sb is larger than 0.15. This result is confirmed by the reflectance spectra measured by using a Fourier transform spectrometer. The analysis of GaAs-like LO lineshape shows that the MBE-grown GaAs_(1-x)Sb_x material used in this work has good quality. The experimen tal results are discussed by using the mass defect model and percolation theory.