In this work the Nusselt number and the heat transfer characteristics of the longitudinal flow through isosceles triangular and rectangular rod bundles are studied analytically. The flow under consideration is steady, laminar and fully developed both thermally and hydrodynamically and the thermophysical properties are constant. Two different boundary conditions are included. They are: (1) uniform heat flux along axial direction and uniform wall temperature along circumferential direction; (2) uniform wall heat flux along both axial and circumferential directions.