The form factor and coupling constant for vertex DDσare calculated. For taking into account the inner structure of D0meson. The Bethe Salpeter amplitude is adopted to describe the heavy meson states. The Mandelstam generalization of the Gell Mann Low methods is applied to calculate the matrix element of quark current between the heavy meson states. The lowest order approximation is taken for the Bethe Salpeter irreducible kernel which is required by the Mandelstam generalization of the Gell Mann Low method. After obtaining the DDσform factor, the DDσcoupling constant is worked out.
