We have investigated the relativistic effects to describe the movement of the shifted emission lines in SS433. The results we have obtained are in agreement with the relativistic theory, developed to study the dispersion of electromagnetic modes in moving outflows and with the kinematic model developed by Morgan et al.(1979) to explain the anomalus emission of the SS443 system. We have found that the dispersion properties of the relativistic plasma, are dependent of the frequency of the emitted radiation, and the refractive index becomes smaller than unity at a narrow frequency range in the far-IR region. In this region, the line Doppler displacement is substantially affected by plasma dispersion as the refrative index becomes smaller than unity. At higher frequencies, the radiation is not affected by these dispersive effects. As a consequence we have found the observed optical and IR Doppler shift ( z ) modulation with the precession time of the SS433 jets is well described by the previous works, but has to be corrected by plasma dispersion effects in the far-IR range.