  For $\beta>0$ and $p\ge 1$, the generalized Ces\`aro operator $$
\mathcal{C}_\beta f(t):=\frac{\beta}{t^\beta}\int_0^t (t-s)^{\beta-1}f(s)ds $$
and its companion operator $\mathcal{C}_\beta^*$ defined on Sobolev spaces
$\mathcal{T}_p^{(\alpha)}(t^\alpha)$ and $\mathcal{T}_p^{(\alpha)}(|
t|^\alpha)$ (where $\alpha\ge 0$ is the fractional order of derivation and are
embedded in $L^p(\RR^+)$ and $L^p(\RR)$ respectively) are studied. We prove
that if $p>1$, then $\mathcal{C}_\beta$ and $\mathcal{C}_\beta^*$ are bounded
operators and commute on $\mathcal{T}_p^{(\alpha)}(t^\alpha)$ and
$\mathcal{T}_p^{(\alpha)}(| t|^\alpha)$. We show explicitly the spectra $\sigma
(\mathcal{C}_\beta)$ and $\sigma (\mathcal{C}_\beta^*)$ and its operator norms
(which depend on $p$). For $1< p\le 2$, we prove that $ \hat{{\mathcal
C}_\beta(f)}={\mathcal C}_\beta^*(\hat{f})$ and $\hat{{\mathcal
C}_\beta^*(f)}={\mathcal C}_\beta(\hat{f})$ where $\hat{f}$ is the Fourier
transform of a function $f\in L^p(\RR)$.
