The study on the application of unstructured grids in solving two-dimensional cylindrical coordinates (r-z) problems is scarce, since one of the challenges is the accurate calculation of the control volumes. In this paper, an unstructured grids- based discretization method, in the framework of a finite volume approach, is proposed for the solution of the convection- diffusion equation in an r-z coordinate. Numerical simulations are presented for the natural convection problem. The numerical results of the proposed method are found to be accurate. The employment of unstructured grids leads to flexibility of the discretization method for irregular domains of any shapes.