under current law, illinois's state income tax rate is a flat rate of 4.95 percent. a flat income tax, which taxes all income levels at the same rate, is required by the current illinois state constitution. illinois is one of 18 u.s. states with a flat income tax or no income tax; 32 other states use graduated income taxes, which tax higher incomes at a higher rate. the last state to switch from a flat state income tax to a graduated state income tax was connecticut in 1996.