For Rayleigh-Bénard convection in a binary fluid mixture, we obtained a traveling-wave state with periodic modulation of spatiotemporal dislocation defects in a rectangular cell by the numerical simulation of full hydrodynamic equations. The investigations show that the creation of pairs of convective rolls triggered by Eckhaus instability leading to this modulation. Because the generating frequencies of rolls at the left wall smaller than the annihilating frequencies of rolls at the right wall, the system soon move out of the stable region, in this way the system oscillates back and forth between the stable and unstable Echkaus wave number band. The properties of heat transfer and fluid mixing also studied too.