A method of verifying a non-volatile memory device to increase the read margin even though a negative verifying voltage is not applied is disclosed. The method of verifying a non-volatile memory device includes coupling a cell string to a bit line precharged to a high level through a sensing node, the cell string being provided between a common source line and the bit line; applying a verifying voltage to a plurality of word lines associated with the cell string; disconnecting the bit line from the sensing node; coupling the common source line to the cell string while the verifying voltage is applied to the word lines, wherein the common source line is applied with a bias voltage higher than a ground voltage; and coupling the bit line to the sensing node so as to detect a level of the bit line.