The onset of double-diffusive convection in a horizontal layer, which is heated and salted from below, of Kelvin-Voigt fluid of order two is studied. The linear instability theory is applied to investigate the influence of salt concentration and viscoelastic parameters on the stability of the system. The critical values of Rayleigh numbers are calculated by utilizing the golden section search in the Matlab platform. The results confirm that for different values of the Kelvin-Voigt parameter and the viscoelastic parameters, the onset of convection is delayed, while the salt concentration advances the onset of convection.