  We determined the chemical and kinematic properties of the Galactic thin and
thick disk using a sample of 307,246 A/F/G/K-type giant stars from the LAMOST
spectroscopic survey and Gaia DR2 survey. Our study found that the thick disk
globally exhibits no metallicity radial gradient, but the inner disk ($R \le 8$
kpc) and the outer disk ($R>8$ kpc) have different gradients when they are
studied separately. The thin disk also shows two different metallicity radial
gradients for the inner disk and the outer disk, and has steep metallicity
vertical gradient of d[Fe/H]/d$|z|$ $=-0.12\pm0.0007$ dex kpc$^{-1}$, but it
becomes flat when it is measured at increasing radial distance, while the
metallicity radial gradient becomes weaker with increasing vertical distance.
Adopting a galaxy potential model, we derived the orbital eccentricity of
sample stars and found a downtrend of average eccentricity with increasing
metallicity for the thick disk. The variation of the rotation velocity with the
metallicity shows a positive gradient for the thick disk stars and a negative
one for the thin disk stars. Comparisons of our observed results with models of
disk formation suggest that radial migration could have influenced the chemical
evolution of the thin disk. The formation of the thick disk could be affected
by more than one processes: the accretion model could play an indispensable
role, while other formation mechanisms, such as the radial migration or heating
scenario model could also have a contribution.
