This paper deals with the laminar-flow heat transfer to a fluid flowing axially between a triangular or a square array of cylinders with a uniform wall temperature. The energy equation in finite-difference form is solved to obtain the axial variation of the cross-sectional temperature distribution, and the numerical results for the local Nusselt number Nuloc and the logarithmic-mean Nusselt number Nulm are presented. From the results, correlating equations suitable for predicting Nuloc and Nulm are derived as functions of the pitch-to-diameter ratio σ and the local Graetz number Gzloc or the Graetz number Gz. It is found that, at the same volume fractions of cylinders ɛ, the heat transfer coefficient for the triangular array is larger than that for the square array, especially for the case ɛ > 0.5.