The third harmonic generation (THG) of a linear cavity Ti:sapphire regenerative amplifier by use of a K3B6O10Cl (KBOC) crystal is studied for the first time.Output power up to 5.9 mW is obtained at a central wavelength of 263 nm,corresponding to a conversion efficiency of 4.5％ to the second harmonic power.Our results show a tremendous potential for nonlinear frequency conversion into the deep ultraviolet range with the new crystal and the output laser power can be further improved.