  We find phase transitions and critical phenomena of the FRW
(Friedmann-Robertson-Walker) universe in the framework of an effective
scalar-tensor theory that belongs to the Horndeski class. We identify the
thermodynamic pressure (generalized force) $P$ of the FRW universe in this
theory with the work density $W$ of the perfect fluid, which is a natural
definition directly read out from the first law of thermodynamics. We derive
the thermodynamic equation of state $P=P(V, T)$ for the FRW universe in this
theory and make a thorough discussion of its $P$-$V$ phase transitions and
critical phenomena. We calculate the critical exponents, and show that they are
the same with the mean field theory, and thus obey the scaling laws.
