The direct sensitization upconversion luminescence phenomenon of Ho(0.5)Yb(5) co-doped oxyfluoride glass was studied. There are two strong upconversion luminescence lines positioned at (544.5 nm, 18,365 cm-1) and (658.5 nm, 15,186 cm-1), which are the transitions of 5S2-->5I8 and 5F5-->5I8. There are others upconversion luminescence lines, which are 5F3-->5I8, 5S2-->5I7 and 5I4-->5I8. An evident characteristic saturation phenomenon is found also. That is all the F-P log-log curves of upconversion fluorescence vs. pumping laser power are rather good straight lines. Its slope will increase with enhancement of pumping laser facula. This is mainly caused by energy diffusion among Yb3+ ions.