A numerical simulation of laminar fluid flow and heat transfer over built-in cylinders in a channel is presented. Effects of cylinders that located in a rectangular channel with constant wall temperature on flow and heat transfer have been investigated by the drag coefficient on cylinders wall, skin-friction factor on channel wall, Strouhal number, pumping factor, Nusselt number, and Performance Index (PI) factor, which denote the heat transfer in terms of the pressure drop. Results are validated by the most reliable published works in the literature. Effects of Reynolds number and blockage ratio (β) for the equilateral triangular cylinder for 120≤Re≤180 and 0.15≤β≤0.55 on flow and heat transfer are investigated with more details. Results indicated that by increasing Re for constant blockage ratio, the drag coefficient, Strouhal number, and Nusselt number increase; but the skin-friction coefficient, pumping factor, and PI factor decrease subsequently. Additionally, with an increase in blockage ratio at constant Re, the drag coefficient, skin-friction coefficient, pumping factor, and Strouhal number grow up; but Nusselt number diminishes and PI factor has an optimum range. Furthermore, results reveal that variation in blockage ratio has more significant effects on the flow and heat transfer than variation in Reynolds number.