  We propose a coupled microdisk laser as a compact and tunable laser source
for the generation of a coherent continuous wave THz radiation by photomixing.
Using the Schr\"odinger-Bloch model including the nonlinear effect of active
medium, we find single mode and two mode lasings depending on the pumping
strength. We explain the transitions of lasing modes in terms of resonant modes
which are the solutions of the Schr\"odinger-Bloch model without active medium
and nonlinear interaction. In particular, a two mode lasing is shown to
generate THz oscillating frequency originating from the light beating of two
nearly degenerated resonant modes with different symmetries.
