Interaction between water and lubrication grease oil is not desirable, but it is not inevitable. The operational life of the machine component is influenced by humid grease oil. Metameterial-based transmission line sensor application is proposed and its operation is validated both numerically and experimentally by inserting grease oil samples in the sample holder. The proposed sensor structure is able to determine the humidity ratio of grease oil samples. The proposed structure shifts resonance frequency to react to the grease oil's dielectric constant. The frequency shift of grease oil samples is monitored as about 300 MHz's. Linearity characteristic between resonant frequency and dielectric constant value is validated. The novelty of this article is that no such study has been conducted by employing non-destructive methods which provide long-term stability of the sensor structure.