Abstract: Based on the definition of Dirac-delta function δ（x）,no definition at x=0 for δ（x）is explained.The δ（x） possesses a dimension of the reciprocal of x and the physical significance of x operator＇s eigenfunction.Some ambiguities in quantum mechanics textbooks are clarified.

Key words: δ function, diagrammatic representation of δ function, dimension of δ function, physical significance of δ function