In this paer, nanoindentation experiment of single crystal aluminum is realized by nano-in-denter. The indentation pattern is scanned by an atomic force microscope (AFM) and hardness value is calculated from it. Hardness property of single crystal aluminum on nano scale is researched emphatically. The results show that the size effect phenomenon exists in nanohardness measurement of single crystal aluminium when the indentation depth is bigger than 2000nm. The essential difference between nanohardness and conventional hardness is discussed; the main reason of it lies in the different emphasis of material property on different scale.