The dispersion curves of phonons along three crystallographic axes for isomorphous crystals NaBrO3 and NaClO3 have been measured by using coherent inelastic neutron scattering. The velocities of elastic waves along these directions and three independently elestic constants C11,C12 and C44 are deduced from the dispersion curves in the region of small wave vectars. The experimental results verified directly the prediction of portigal's theory that the velocities of left circularly polarized phonons and the right polarized ones distribute on both side of a particular average velocity v(TD) symmetrically, the value of v(TD) can be determined uniquely by the velocities on other directions or the zero-order approximative elastic constants.