Abstarct: The purpose of this thesis is to present a method for measuring the quality of transformer oil using online microwave sensors. Transformer oil plays the role of insulation in high-power equipment, such as transformers. The deterioration of these oils leads to the failure of transformers and power distribution networks. Therefore, early detection of the quality of these oils prevents damage to the power distribution network. The fundamental operation of the proposed microwave sensor in this thesis is to measure oil quality baxsed on changes in its dielectric constant and loss tangent. In addition to introducing a completely passive sensor structure, this method can be used as a continuous and online monitoring method for transformer oil without the need for sampling operations. The seven proposed sensors in this research include antenna-baxsed structures, waveguide structures, mextamaterial, and filter. The presentation of various structures has been done with the aim of examining the advantages and disadvantages of each structure compared to mextamaterial structures. Finally, by examining the sensitivities of the proposed structures, high-sensitivity structures, including structures baxsed on gap waveguide resonator, SRR resonator, and the spurline loaded with an impedance stub, were introduced as the final structures. The sensitivities of the final sensors for a 0.08 difference in the permittivity coefficient of clear and dirty transformer oil, respectively are 0.74, 1.41, and 1.95. The final sensors demonstrate high sensitivity in evaluating the quality of transformer oil.