Let $γ(G)$ and $i(G)$ be the domination number and the independent domination number of $G$, respectively. In 1977, Hedetniemi and Mitchell began with the comparison of of $i(G)$ and $γ(G)$ and recently Rad and Volkmann posted a conjecture that $i(G)/ γ(G) \leq Δ(G)/2$, where $Δ(G)$ is the maximum degree of $G$. In this work, we prove the conjecture for trees and provide the graph achieved the sharp bound.