  We conjecture that the phase transitions in QCD at large number of colours
N\gg 1 is triggered by the drastic change in the instanton density. As a result
of it, all physical observables also experience some sharp modification in the
\theta behaviour. This conjecture is motivated by the holographic model of QCD
where confinement -deconfinement phase transition indeed happens precisely at
temperature T=T_c where $\theta$ dependence of the vacuum energy experiences a
sudden change in behaviour: from N^2\cos(\theta/N) at T<T_c to
\cos\theta\exp(-N) at T>T_c. This conjecture is also supported by recent
lattice studies. We employ this conjecture to study a possible phase transition
as a function of \kappa\equiv N_f/N from confinement to conformal phase in the
Veneziano limit N_f\sim N when number of flavours and colours are large, but
the ratio \kappa is finite. Technically, we consider an operator which gets its
expectation value solely from nonperturbative instaton effects. When \kappa
exceeds some critical value \kappa> \kappa_c the integral over instanton size
is dominated by small-size instatons, making the instanton computations
reliable with expected \exp(-N) behaviour. However, when \kappa<\kappa_c, the
integral over instaton size is dominated by large-size instantons, and the
instanton expansion breaks down. This regime with \kappa<\kappa_c corresponds
to the confinement phase. We also compute the variation of the critical
\kappa_c(T, \mu) when the temperature and chemical potential T, \mu \ll
\Lambda_{QCD} slightly vary. We also discuss the scaling
(x_i-x_j)^{-\gamma_{\rm det}} in the conformal phase.
