The solid-state boriding of new damage tolerance TC21 titanium alloy was carried out to harden surface layers through rare earth (RE) B diffusion process. By means of SEM, XRD, OM and EDS, the microstructure and morphology of boride coatings were investigated. The results indicate that boride layers include a monolithic titanium diboride (TiB2) at the top and a titanium boride (TiB) sub-layer that mostly consisted of TiB whiskers combining with titanium matrix closely. Meanwhile, the contents of RE in boriding medium possess a considerably optimal value. B and C surface co-diffusion with RE on TC21 alloy can be realized by this method