Let $p=(q^4+q^3+q^2+q+1)/(5,q-1)$ be a prime number, where $q$ is a prime
power. In this paper, we will show $Gcong mathrm{PSL}(5,q)$ if and only if
$|G|=|mathrm{PSL}(5,q)|$, and $G$ has a conjugacy class size $frac{|
mathrm{PSL}(5,q)|}{p}$. Further, the validity of a conjecture of J. G.
Thompson is generalized to the groups under consideration by a new way.