  Recent observations demonstrate that the thin and thick disks of the Galaxy
have different chemical abundance trends and evolution timescales. The relative
abundances of $\alpha$-elements in the thick Galactic disk are increased
relative to the thin disk. Our goal is to investigate the cause of such
differences in thick and thin disk abundances. We investigate the chemical
evolution of the Galactic disk in the framework of the open two-zone model with
gas inflow. The Galactic abundance trends for $\alpha$-elements (Mg, Si, O) and
Fe are predicted for the thin and thick Galactic disks. The star formation
histories of the thin and thick disks must have been different and the gas
infall must have been more intense during the thick disk evolution that the
thin disk evolution.
