  We investigate the permutation property of polynomials of the form
$x^{r}(x^{s} -a)^{t}$, and give the expressions of their inverses. In
particular, explicit expressions of inverses of permutation polynomials $x(x^3
-a)^2$ and $x(x^2 -a)^3$ on $\mathbb{F}_{7^n}$ are presented. Then, using some
known results, we obtain the inverses of all permutation polynomials of degree
$6, 7, 8$ over finite fields.
