  We study the radiative electroweak symmetry breaking and the relic abundance
of neutralino dark matter in the supersymmetric type I seesaw model. In this
model, there exist threshold corrections to Higgs bilinear terms coming from
heavy singlet sneutrino loops, which make the soft supersymmetry breaking (SSB)
mass for up-type Higgs shift at the seesaw scale and thus a minimization
condition for the Higgs potential is affected. We show that the required
fine-tuning between the Higgsino mass parameter mu and the SSB mass for up-type
Higgs may be reduced at the electroweak scale, due to the threshold
corrections. We also present how the parameter mu depends on the SSB
B-parameter for heavy singlet sneutrinos. Since the property of neutralino dark
matter is quite sensitive to the size of mu, we discuss how the relic abundance
of neutralino dark matter is affected by the SSB B-parameter. Taking the SSB
B-parameter of order of a few hundreds TeV, the required relic abundance of
neutralino dark matter can be correctly achieved. In this case, dark matter is
a mixture of bino and Higgsino, under the condition that gaugino masses are
universal at the grand unification scale.
