Three-dimensional laminar steady fluid flow and conjugate heat transfer from an array of rectangular solid and perforated fins are presented numerically. Perforations with window square cross section in various numbers and dimensions are arranged in the lateral surface of fins. Due to perforations, the flow may become unsteady sooner than solid fins; hence, the computation performed for the range of Reynolds numbers 100 to 250. For analysis, a FORTRAN code base on the SIMPLE algorithm with staggered grid is developed. Also, the second-order upwind technique is used to calculate convective terms in momentum and energy equations. Results show that by utilizing perforated fins, considerable fin weight reduction is achieved without any penalty for heat transfer rate.