Context : 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 .

Aims : 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 .

Methods : 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 .

Results : 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 northwest 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 emission , 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 .

Conclusions : The Monoceros SNR is interacting with the ambient interstellar medium with ultra-high energy emission detected .
Its interaction with the Rosette Nebula is further supported by new evidence from HI data , which will help the investigation of the emission mechanism of the high energy emission .