This paper presents a new, and efficient, wear debris detection sensor based on passive wireless LC sensing approach. This sensor enables online monitoring of metal debris in hydraulic flow tubes with large thickness, employed in high-pressure applications. In conventional inductive debris sensors, the coil is kept outside the tube. (i) Hence, for thick tubes, the sensitivity is extremely low. If the coil is kept inside the tube, holes are required to connect it to the measurement unit (MU). (ii) This limits the pressure withstanding capacity. The proposed sensor solves these two problems using the wireless LC sensor approach, wherein the MU is connected to an outer coil (outside the thick tube), and an inner coil is kept close to the oil, with debris. Thus, it is highly sensitive to the debris. It requires no electrical connection to the MU. The outer coil is energized from an AC signal at the parallel resonance frequency of the system, drawing minimum current from the source. The change in phase of this current, due to the change in inductance of the inner coil as debris passes, is measured using a phase-sensitive detector. This highly sensitive measurement approach together with the fact that the inner coil is close to the oil, helps to achieve high sensitivity, and enables monitoring flow tubes with large diameter and thickness. A prototype of the proposed wireless LC debris sensor has been developed (inner and outer diameters of 25.4 mm and 80 mm respectively), integrated into a hydraulic circuit and tested. The output was not only very sensitive to ferrous and non-ferrous metallic particles but also helps in distinguishing them. © 2001-2012 IEEE.