  The dark energy cosmology with the equation of state $w=constant$ is
considered in this paper. The $\Omega_{DE}-\Omega_{M}$ plane has been used to
study the present state and expansion history of the universe. Through the
mathematical analysis, we give the theoretical constraint of cosmological
parameters. Together with some observations such as the transition redshift
from deceleration to acceleration, more precise constraint on cosmological
parameters can be acquired.
