A method of operating a semiconductor device includes applying a first voltage to a first source select line coupled to first source select transistors of memory strings included in an unselected memory block, among memory blocks, floating the first source select line after the first voltage is applied thereto, applying a second voltage having a lower voltage level than the first voltage to a second source select line coupled to second source select transistors of the memory strings included in the unselected memory block, applying a precharge voltage to a common source line, and applying a program voltage to a word line coupled to selected memory cells of memory strings included in a selected memory block, among the memory blocks.