  An improved leading order fragmentation functions set of kaon is proposed
based on the experimental data. We compare it with currently available sets,
and use it to calculate high-$p_T$ $K/\pi$ ratios in relativistic proton-proton
collisions. A prediction of $K/\pi$ ratios in relativistic nucleus-nucleus
collisions ($d + Au$ and $Au + Au$) at RHIC $\sqrt{s} = 200$ GeV is given.
