In this talk I will present our study of the (UV-U)rest and (U-R)rest colour gradients of a sample of 17 elliptical galaxies morphologically selected in the cluster XMMU J2235.3-2557 at z=1.39 in order to constrain the mechanisms by which elliptical galaxies aggregate their stellar mass. The analysis is based on HST archive data composed of optical observations obtained with the ACS camera in the F775W and F850LP bands and of near-IR observations obtained with the WFC3 camera in the F160W band. We detected positive UV-U (bluer inwards) colour gradients and negative (redder inwards) U-R colour gradients for all the galaxies of the sample. I will discuss the nature of the observed colour gradients. The analysis is based on the comparison with the synthetic stellar population models of Bruzual & Charlot (2003). I will also present a comparison between colour gradients in cluster and field elliptical galaxies selected in a comparable range of redshift in order to determine the role of the environment in the assembly of the stellar mass in elliptical galaxies. Finally, through the evolution of the observed colour gradients up to z=0, I will investigate the evolution of the stellar populations properties of the studied galaxies.