Let $(X,\delta,\mu)$ be a normal space of 

homogeneous type of order $\gamma$.

Gatto and V\'agi \cite{GV2} showed that, 

if $f$ and $I_\alpha f$ are in $L^p(X)$

$(0<\alpha<\min(\gamma,1/p))$,

then $I_\alpha f$ is in $C^{p,\alpha}(X)$, 

where $I_\alpha$ is the Riesz potential of 

order $\alpha$ and $C^{p,\alpha}$ is the 

space of smooth functions of Calder\'on-Scott 

\cite{CS}.

In this paper, we introduce a generalized 

Riesz potential $I_\phi$ and extend the result above. 

With this aim, we extend the Hardy-Littlewood-Sobolev 

inequality to the Orlicz space.