Abstract: We studied electron single-slit, double-slit and three-slit diffraction with a new quantum mechanics approach and obtained the following results. (1) For single-slit diffraction, the thickness of the single-slit can make a large impact on the diffraction intensity and pattern. (2) For double-slit diffraction, when the ratio of (a+d)/a=n(n=1,2,3,…), orders n,2n,3n,… are missing in diffraction pattern, when the ratio of (a+d)/a≠n(n=1,2,3,…), there isn’t missing order in diffraction pattern. (3) For double-slit diffraction, it can be found that when the thickness of the double-slit increases, the diffraction intensity and pattern will be changed, and when the thickness of the double-slit increases, the missing order phenomena disappear. (4) For three-slit diffraction, when the thickness of the slit increases, the diffraction intensity and pattern will be changed, and there is one secondary maxima and two minima between the two principle maxima.

Key words: matter-wave, diffraction, quantum theory