A physically unclonable function (PUF) is a promising technology that provides the ability to identify network components. Conventional PUF circuitry suffers from a reliability problem in which a consistent output value is not guaranteed because of environmental changes such as the power supply voltage and operating temperature. This paper introduces a PUF circuit that yields consistent responses that overcome fluctuations in the operating conditions. The proposed PUF is based on the variation in the crosstalk between adjacent transmission lines, which reflects the random physical structure of interconnected lines. The proposed PUF cell comprises three transmission lines and a sense amplifier. The sense amplifier catches the crosstalk differences between two adjacent transmission lines. The crosstalk differences reflect a physical structure variation, which is immune to changes in the operating environment. The proposed PUF circuit yields a well-balanced distribution probability, (100/2^n)%, for each challenge bit with Sigma=1%. The identification generated by the PUF can be regarded as core functionality for authentication and encryption in security applications.