  We constrain holographic dark energy (HDE) with time varying gravitational
coupling constant in the framework of the modified Friedmann equations using
cosmological data from type Ia supernovae, baryon acoustic oscillations, cosmic
microwave background radiation and X-ray gas mass fraction. Applying a Markov
Chain Monte Carlo (MCMC) simulation, we obtain the best fit values of the model
and cosmological parameters within $1\sigma$ confidence level (CL) in a flat
universe as: $\Omega_{\rm b}h^2=0.0222^{+0.0018}_{-0.0013}$, $\Omega_{\rm c}h^2
=0.1121^{+0.0110}_{-0.0079}$, $\alpha_{\rm G}\equiv \dot{G}/(HG)
=0.1647^{+0.3547}_{-0.2971}$ and the HDE constant
$c=0.9322^{+0.4569}_{-0.5447}$. Using the best fit values, the equation of
state of the dark component at the present time $w_{\rm d0}$ at $1\sigma$ CL
can cross the phantom boundary $w=-1$.
