Heat transfer near the critical condition of Carbon Dioxide due to thermo-acoustic waves in a 100-µm high microchannel was numerically studied. The temperature at a point farthest away from the heated surface was compared between computational fluid dynamics (CFD) models and a pure conduction model. The comparison revealed that the CFD model predicted a temperature increase furthest from the surface much faster than the time constant required for such increase purely by conduction. It is believed that another heat transfer process, termed the piston effect (PE), which is associated with pressure waves in the fluid, was responsible for this increase. Explicit unsteady methodology in the fluid model indicated that propagation of pressure waves due to a rapid expansion of the boundary layer and the associate change in the fluid density distribution resulted in this temperature raise. It was confirmed that natural convection wasn’t responsible for the temperature increase under quiescent conditions. In addition, it was discovered that the PE is significant for certain forced convection conditions.