This paper proposes a novel CMOS built-in current sensor (BICS) for on-line current testing. Proposed BICS detects abnormal current in circuit under test (CUT) and makes a Pass/Fail signal through comparison between the CUT current and the duplicated inverter current. This circuit consists of two current-to-voltage conversion transistors, a full swing generator, a voltage comparator, and an inverter block. It requires 16 transistors. Since this BICS does not require the extra clock, the added extra pin is only one output pin. Furthermore, the BICS does not require test mode selection. Therefore the BICS can be applied to on-line current testing. The validity and effectiveness are verified through the HSPICE simulation of circuits with defects. When the CUT is an 8 8 parallel multiplier, the area overhead of the BICS is about 4.34%.