© 2018 Author(s). Impact of processing-induced structural defects on the electrical properties of a graphene monolayer has been investigated using scanning microwave microscopy (SMM). Graphene sheets grown on copper foil by chemical vapor deposition were transferred to a silicon wafer covered with a 300 nm thick thermal SiO2 layer and then patterned into a grating structure using the standard lithography technique. Raman spectroscopy and SMM were employed to monitor the defect generation and the induced surface impedance change on graphene. Correlation of the SMM image contrast shows that the etching-induced defects cause a decrease of the electrical conductivity and permittivity of the graphene monolayer. In addition, the SMM image contrast shows a frequency dependency: at higher frequencies, the permittivity of the graphene monolayer plays an important role, resulting in the SMM phase imaging contrast reversed from the low frequency measurements. Numerical simulations were performed, which are in very good agreement with the experimental results.