We find phase transitions and critical phenomena in cosmic thermodynamics. We derive the thermodynamic pressure (generalized force) P of the FRW (Friedmann–Robertson–Walker) universe in an effective higher order scalar-tensor theory with the work density W of a perfect fluid, which is a natural definition directly read out from the first law of thermodynamics. By this identification, we further construct an equation of state P=P(V,T) for the FRW universe and make a thorough discussion of its P−V phase transitions and critical phenomena. Impressively, we find that the coexistence phases of the phase transitions appear above the critical temperature, which is different from that of van der Waals system and most of black holes system. In the end, we calculate the critical exponents near the criticality of FRW universe and show that they are universal and are independent of the details of the physical system although the thermodynamic quantities depend on the coupling parameters.