Abstract: This paper gives an useful result about rectangles,i.e.the result of Th.2.And it shows that the two-order constant coefficient equations(1) for two independent variables,two unknown functions can be transformed into strongly elliptical difference equations.And this resultcan also be extended to the case of some three unknown functions.By use of the result that the strongly elliptical equations has the uniqueness of the solution,it is proved that the uniqueness of the solution about the Dirichlet problem for some linear elliptical partial difference equations of two-order in a bounded domain.