The calculation of the elastic scattering properties of mixtures composed of ultracold Rb-K atoms is reported and compared with experimental results in detail. The improved potentials for both the singlet and triplet molecular states of Rb-K are presented, and the scattering lengths at and the effective range re are calculated with numeric and semiclassical methods for Rb-K in the triplet and singlet state. In addition, the convergence of these scattering properties depending on a K0 parameter is investigated using quantum defect theory. Other results of the evaporative cooling that include the cross section and the spin-charge cross section, the rate coefficient as a function of the energy and ultralow temperature are also presented in this study. ©2006 The Physical Society of Japan.