This work experimentally studied the pressure drop and heat transfer of a square pin-fin array in a rectangular channel by using the transient single-blow technique. The variable parameters are the relative longitudinal pitch (X-L = 1.5, 2, 2.8), the relative transverse pitch (X-T = 1.5, 2, 2.8) and the arrangement (in-line or staggered). Compared with the open articles, the present relative pitches are smaller and independently variable. The performance of the square pin-fins as the cooling devices is compared with that of the circular pin-fins. Besides, empirical formulas for the pressure loss and the heat transfer are suggested. Finally, the optimal inter-fin pitches are provided based on the largest Nusselt number under the same pumping power, while the optimal inter-fin pitches of square pin-fins are X-T = 2 and X-L = 1.5 for the arrays in in-line arrangements as well as X-T = 1.5 and X-L = 1.5 for the arrays in staggered arrangements. (c) 2006 Elsevier Ltd. All rights reserved.