  The effects of conformal anomaly on the thermodynamics of black holes are
investigated in this Letter from the perspective of $P-V$ criticality of AdS
black holes. Treating the cosmological constant as thermodynamic pressure, we
extend the recent research to the extended phase space. Firstly, we study the
$P$-$V$ criticality of the uncharged AdS black holes with conformal anomaly and
find that conformal anomaly does not influence whether there exists Van der
Waals like critical behavior. Secondly, we investigate the $P$-$V$ criticality
of the charged cases and find that conformal anomaly influences not only the
critical physical quantities but also the ratio $\frac{P_cr_c}{T_c}$. The ratio
is no longer a constant as before but a function of conformal anomaly parameter
$\tilde{\alpha}$. We also show that the conformal parameter should satisfy a
certain range to guarantee the existence of critical point that has physical
meaning. Our results show the effects of conformal anomaly.
