  Given a connected and bridgeless graph $G$, let $\mathscr{D}(G)$ be the
family of strong orientations of $G$. The orientation number of $G$ is defined
to be $\bar{d}(G):=min\{d(D)|D\in \mathscr{D}(G)\}$, where $d(D)$ is the
diameter of the digraph $D$. In this paper, we focus on the orientation number
of complete tripartite graphs. We prove a conjecture raised by Rajasekaran and
Sampathkumar. Specifically, for $q\ge p\ge 3$, if $\bar{d}(K(2,p,q))=2$, then
$q\le{{p}\choose{\lfloor{p/2}\rfloor}}$. We also present some sufficient
conditions on $p$ and $q$ for $\bar{d}(K(p,p,q))=2$.
