An operation method of a non-volatile memory suitable for a multi-level cell having a first storage position and a second storage position is provided. The operation method includes: setting a main voltage distribution group and a plurality of secondary voltage distribution groups, wherein each of the main voltage distribution group and the secondary voltage distribution groups includes N threshold-voltage distribution curves, and N is an integer greater than 2; selecting a first operation level and a second operation level according to a programming command; programming the first storage position according to the threshold-voltage distribution curve corresponding to the first operation level in the main voltage distribution group; selecting one of the secondary voltage distribution groups according to the first operation level and programming the second storage position according to the threshold-voltage distribution curve corresponding to the second operation level in the selected secondary voltage distribution group.