fact a vector sum of different polarized sub-components, the percentage polarization calculated in this way represents a lower limit. We can see from the image (Figure \[fig:MERpol\]) that the percentage of the polarization at individual pixels is higher than 5 per cent, and increases toward the south of the nucleus. A maximum value of $m\gtrsim30$ per cent is detected at $\sim$0.045 arcsec south of the nucleus. The fractional polarization ($m>4.9$ per cent) measured from our MERLIN observation at 18 cm is at least an order of magnitude higher than the VLA measurement at 20 cm, although it is consistent with the values measured by the VLA at 6 cm and shorter wavelengths. This difference in the fractional polarization at these very similar wavelengths is most likely to be an observational effect due to beam depolarization, rather than being due to intrinsic variations in the Faraday depth (R. Perley, private communication).

The averaged polarization angle (or EVPA) is $-18\degr\pm5\degr$ in the polarization structure. On the basis of the new measurements of the Rotation Measure (RM) towards 3C 48 by Mantovani et al. (2009), i.e., RM=$-64$ rad m$^{-2}$ and intrinsic position angle $\phi_0=116\degr$ [@Sim81; @Man09], we get a polarization angle of $-4\degr$ 