The effect of nonmagnetic impurity scattering on the additional damping of de Haas-van Alphen (dHvA) oscillations in the mixed state of type II superconductors near the upper critical field is studied within mean field theory in the relaxation time approximation. Our calculations show that this effect becomes important only for small thermal smearing of the single particle distribution function and small disorder broadening of the Landau levels. The relationships between the different mechanisms of broadening are discussed.