  The Monoceros Loop (SNR G205.5+0.5) is a large shell-type supernova remnant
located in the Rosette Complex region. It was suggested to be interacting with
the Rosette Nebula. We aim to re-examine the radio spectral index and its
spatial variation over the Monoceros SNR, and study its properties of evolution
within the complex interstellar medium. We extracted radio continuum data for
the Monoceros complex region from the Effelsberg 21 cm and 11 cm surveys and
the Urumqi 6 cm polarization survey. We used the new Arecibo GALFA-HI survey
data with much higher resolution and sensitivity than that previously available
to identify the HI shell related with the SNR. Multi-wavelengths data are
included to investigate the properties of the SNR. The spectral index $\alpha$
($S_{\nu}\propto\nu^{\alpha}$) averaged over the SNR is $-0.41 \pm$0.16. The
TT-plots and the distribution of $\alpha$ over the SNR show spatial variations
which steepen towards the inner western filamentary shell. Polarized emission
is prominent on the western filamentary shell region. The RM there is estimated
to be about 30$\pm$77n rad m$^{-2}$, where the n=1 solution is preferred, and
the magnetic field has a strength of about 9.5 $\mu$G. From the HI channel
maps, further evidence is provided for an interaction between the Monoceros SNR
and the Rosette Nebula. We identify partial neutral hydrogen shell structures
in the northwestern region at LSR velocities of +15 km s$^{-1}$ circumscribing
the continuum emission. The HI shell has swept up a mass of about 4000
M$_{\odot}$ for a distance of 1.6 kpc. The western HI shell, well associated
with the dust mission, is found to lie outside of the radio shell. We suggest
that the Monoceros SNR is evolving within a cavity blown-out by the progenitor,
and has triggered part of the star formation in the Rosette Nebula.
