Published online by Cambridge University Press: 20 November 2018
The unitary Cayley graph of a ring $R$, denoted $\Gamma \left( R \right)$, is the simple graph defined on all elements of $R$, and where two vertices $x$ and $y$ are adjacent if and only if $x\,-\,y$ is a unit in $R$. The largest distance between all pairs of vertices of a graph $G$ is called the diameter of $G$ and is denoted by $\text{diam}\left( G \right)$. It is proved that for each integer $n\,\ge \,1$, there exists a ring $R$ such that $\text{diam}\left( \Gamma \left( R \right) \right)=n$. We also show that $\text{diam}\left( \Gamma \left( R \right) \right)\in \left\{ 1,2,3,\infty \right\}$ for a ring $R$ with ${R}/{J\left( R \right)}\;$ self-injective and classify all those rings with $\text{diam}\left( \Gamma \left( R \right) \right)\,=\,1,\,2,\,3$, and $\infty$, respectively.