A numerical analysis is performed to study the mechanism of energy separation in a viscous heat-conducting shear layer. Two-dimensional time-dependent Navier-Stokes equations and total energy conservation equation are solved simultaneously for four different Reynolds numbers: 100, 200, 500, and 1000. The results show that the roll-up and transport of vortices induce pressure fluctuations in the shear layer. Fluid which flows through the disturbed pressure field exchanges pressure work with the surroundings and separates into higher and lower total temperature regions. Even though some energy separation is caused by the imbalance between shear work and heat conduction, that due to the pressure fluctuation is much stronger. As the Reynolds number is increased, turbulence mixing between higher and lower total temperature regions tends to lessen the energy separation. However, the more rapid growth of pressure fluctuations overcomes the weakening of energy separation by random mixing.