A hydraulic oil micro sensor is designed, which is based on the microfluidic chip technology. The microsensor consists of an inductive sensor embedded in silicon planar coils opposing silicon. Metal particles are detected in the inductance detection mode. Water droplets and bubbles are detected in the capacitance mode. Four kinds of pollutants in the oil can be distinguished, which are ferromagnetic particles, non-ferromagnetic particles, water droplets and bubbles. Experiments verify that the new structure of the micro sensor improves the detection accuracy of metal particles in the inductive mode. It can detect 28 μm iron particles and 85 μm copper particles in the oil. In the capacitance mode, the micro sensor has the best detection effectiveness when the excitation voltage is 2 V and the excitation frequency is 0. 9 MHz. In addition, it can detect 95 μm water droplets and 160 μm bubbles in the oil. This research is great significant for failure prediction and diagnosis of hydraulic systems.