  We study cosmological effects of homogeneous tachyon field as dark energy. We
concentrate on two different scalar field potentials, the inverse square
potential and the exponential potential. These models have a unique feature
that the matter density parameter and the density parameter for tachyons remain
comparable for a large range in redshift. It is shown that there exists a range
of parameters for which the universe undergoes an accelerated expansion and the
evolution is consistent with structure formation requirements. For a viable
model we require fine tuning of parameters comparable to that in LCDM or in
quintessence models. For the exponential potential, the accelerated phase is
followed by a phase with a(t)~ t^{2/3} thus eliminating a future horizon.
