We show that $b \in BMO(\R^n)$ if and only if the commutator
 $[b,I_{\alpha}]$  of the multiplication operator by
 $b$ and the fractional integral operator $I_{\alpha}$ is bounded
 from generalized Morrey spaces $L^{p,\varphi}(\R^n)$ to 
 $L^{q,\varphi^{q/p}}(\R^n)$, where $\varphi$ is  non-decreasing, and
 $1<p< \infty$, $0<\alpha <n$ and $1/q=1/p-\alpha /n$.