We propose a microwave sensor on the basis of complementary circular spiral resonator (CCSR) for substrate determination and thickness identification in this paper. The developed sensor consists of a microstrip transmission line and CCSR. By taking advantage of High Frequency Structure Simulator (HFSS), the designed device operates at 2.9 GHz with high Q factor. Through simulation, a strong electric field is generated besides CCSR, making this region very susceptible to the transformation in nearby dielectric substrate. When different substrates to be tested are put on sensing area, resonance frequencies change greatly. The designed device can also be used for thickness determination of a particular substrate. Two parabolic functions are calculated and fitted to evaluate relative permittivity of unknown material to be tested and the thickness of FR4 substrate. The sensor has many advantages, such as low cost, easy operation, high sensitivity and robustness, making it popular in substrates dielectric characterization.