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Multi-frequency Very Long Baseline Array (VLBA) polarimetry observation ofthe GHz-Peaked Spectrum (GPS) quasar OQ172 (J1445+0958) has been performed at1.6, 2.2, 4.8, 8.3 and 15.3 GHz in 2005. Core-jet structures are detected inall bands with the jet strongly bent at about 3 mas from the core. The radioemission of the source is polarised at all five bands. We study the FaradayRotation in the core and jet components at all five bands, and find good linearfits of Faraday Rotation in the core and jet components at 4.8 and 8.3 GHz. Atthese two frequencies, the Rotation Measure (RM) is $\sim 2000~\rm rad~m^{-2}$in the core and $\sim 700~\rm rad~m^{-2}$ in the inner jet components andcontinues to decrease at the outer jet parts. We find that the depolarisationat 4.8 and 8.3 GHz might be caused by the internal medium in the source. Weinvestigate consistency of the turnover spectra of VLBI components with theSynchrotron Self-Absorption (SSA) and Free-Free Absorption (FFA) models.Although these two models can not be easily distinguished due to the lack oflow-frequency data, the physical parameters can be constrained for each model.We find that the large width of the $\rm [OIII]_{5007}$ line is likely causedby a jet interaction with a Narrow Line Region (NLR) medium. The jet bending,significant RM variations, Faraday depolarisation, spectral turnover, and broadline width of $\rm [OIII]_{5007}$ could be closely related, likely caused bythe same nucleus medium, presumably NLR.

Multi-frequency VLBA Polarimetry of the high-redshift GPS Quasar OQ172