Synchrotron radiation from AGN jets can be linearly polarized up to ∼70% in theory. However, the observed degree of linear polarization is usually much lower. The weak linear polarization could to be due to (i) highly tangled magnetic fields in turbulent plasma, (ii) strong Faraday rotation and (iii) blending of polarized sub-components within the beam. The bright radio-galaxy 3C 84 (Perseus A), which is located at the center of the Perseus Cluster, is one of most weakly polarized radio galaxies known in cm-VLBI, featuring a complicated sub-structure. We performed deep polarization VLBI imaging using the Global mm-VLBI Array (GMVA) at 86 GHz. This provides new images with a very high angular and spatial resolution of 50uas (250Rs), which provides further inside at the galaxy center, in regions where the two-sided VLBI jet is formed. Here we report the first time detection of substantial linear polarization in the VLBI core region at ~10sigma significance level with VLBI at 86 GHz. In this talk we provide a physical interpretation of the polarization properties in 3C84 in relation to earlier results from polarimetry performed at other wavelength and with other instruments.