Method and apparatus of identifying ink stored in an ink-jet cartridge are to change the electrical property such as resistance or capacitance of the ink and to cooperate with a detector to achieve the goal of identifying ink. The method comprises: adding special additive such as salt additive or acid additive into the ink or changing the ratio of the components in the ink, and further providing a detector for detecting the electrical property of the ink and doing logical judgment to identify whether the ink stored in the ink-jet cartridge is the original factory ink or not. The preferred embodiment of the apparatus of identifying ink in ink-jet cartridge is to form the detector on the IC of the print head directly to achieve the goal stating above of detecting the electrical property of the ink stored in the ink-jet cartridge.