This paper investigates numerically the influence of detached square vortex generator (VGs) on the heat transfer and pressure drop inside a square duct. Reynolds number is fixed at 5000. The geometrical parameters in this investigation are: i) The blocking ratios are 0.1, 0.15 and 0.2), ii) Vortex generator numbers are 1, 2, and 3), iii) Attack angles are 0, 30, and 45, iv) The aspect ratios are 1, 1.5 and 2. The numerical simulation is carried out using ANSYS FLUENT 15. The results show that the rectangular vortex generators have a positive influence on heat transfer as a result of the augmentation in turbulence level. The maximum enhancement in average heat transfer could reach 40%. The heat transfer is found to increase with the blocking ratio. The heat transfer enhanced by 17% for one VG and 28% for 3 VGs for blocking ratio = 0.2. The VGs at angle value of 45° produce the highest heat transfer enhancement. The aspect ratio is found to have an adverse effect on heat transfer rate. Cite this article as: Mustafa J. A, Fadhil A K, Hamad F A. Numerical investigation of the heat transfer enhancement inside a square duct with rectangular vortex generators.