  Using a geometric description of 2HDM, we classify CP invariants into three
independent sectors such as scalar potential, Yukawa interaction and CKM
matrix. Thermal effective potential of 2HDM is calculated in a basis invariant
way. It is shown that the CP violation in Yukawa interactions can contribute to
effective potential at one loop level but the CP phase in the CKM matrix cannot
leak to effective potential at all orders. In the 2HDM with a softly broken Z_2
symmetry, the leading thermal correction tends to restore the CP symmetry at
high temperature.
