  In this paper, motivated from the fact that a de Sitter cosmological boundary
corresponds to a positive cosmological constant, we consider time variable
cosmological constants, dubbed {\it horizon cosmological constants}. The
horizon cosmological constants correspond to Hubble horizon, future event
horizon and particle horizon are discussed respectively. When the Hubble
horizon is taken as a cosmological length scale, the effective equation of
state of horizon cosmological constant is quintessence-like. The values of
model parameter $c$ will determine the current status of our universe. When
particle horizon is taken as the cosmological length scale, non viable
cosmological model can be obtained for the requirement of
$\Omega_{\Lambda}<1/3$ which conflicts with current comic observations. When
the future event horizon is taken as the role of cosmological length scale, the
forms of effective equation of state of horizon cosmological constants are the
the same as the holographic ones. But, their evolutions are different because
of the effective interaction with cold dark matter.
