The isotope effect has been measured for surface diffusion of potassium isotopes on polycrystalline tungsten using ion thermoemission noise method. It has been noted that the isotopic ratio41K/39K obtained by measurement of noise component is different from isotopic ratio for DC component. This shift was interpreted as isotope fractionation effect in surface diffusion of adsorbed potassium. It was found that the spectral density function of the ion current fluctuation can be approximate by the Lorentzian function S = S0/1 + (ωτ)2.