Two- and three-dimensional unsteady flow and heat transfer around a square cylinder have been investigated using Ansys Cfx software. For reducing the forces and their fluctuations, an active control with the use of front and rear jets separately and simultaneously were applied. It should be noted that the temperature of the jet nozzle was considered to be same as that of cylinder temperature. All simulations were carried out at a Reynolds number (Re) of 22000 and Prandtl number of 0.7. The unsteady two-dimensional results were provided based on k-w SST turbulence model, while three-dimensional results were obtained using large eddy simulation (LES). As a validation case, the results for cylinder with and without flow control were compared with those of experimental, DNS and LES reported results and a reasonable agreement was observed. In the first part via two-dimensional simulations, with the use of front and rear jets separately and simultaneously, optimal flow control modes were found. The results showed that in the jet blow from the rear of the cylinder, the lowest amount of drag coefficient, the lowest Nusselt number, and the highest time-averaged vortices length behind the cylinder occurs at injection rate of IR = 1.5; and then by increasing its injection rate, the drag coefficient and Nusselt number increase. In addition, at all jet injection rates from the rear of the cylinder, the jet tilts upward and downward frequently, whereas in this case, the time-averaged flow around the cylinder is symmetric. For the rear jet injection in the range of IR 1 and IR 3, there is a pair of time-averaged vortices behind the cylinder with one dominant frequency, while two pairs of time-averaged vortices with two dominant frequencies are observed for 1 IR 3. It was also observed that the jet injection from the rear surface does not have a significant effect on the Nusselt number of the front surface. In the case of jet injection from the rear surface, the Nusselt number of the top, bottom, and rear surfaces are lowest at IR=1.5. In front jet injection case, at injection rates of less than IR = 1, the jet fluctuates simultaneously up and down, but at injection rates greater than IR = 1, the jet is deflected to one side and an averaged non-zero lift force is achieved and the drag force is greatly reduced. In general, the front jet is effective in reducing the drag coefficient and rear jet is effective in reducing the drag coefficient and rms fluctuations. By simultaneously applying the front and rear jet injection, the drag force and the resultant forces of the drag and lift are much less than applying each jet separately. According to the results of two-dimensional simulation, an optimum condition is introduced at IR=1.5. In the three-dimensional study, in addition to the uncontrolled mode, three control modes includes front jet, rear jet and front-rear jet at IR = 1.5 were investigated. In case of the rear jet, the time-averaged results of LES were relatively close to the results of 2D simulation. In case of the front jet, there was a large difference in the prediction of flow patterns and rms oscillations compared to the two-dimensional results. The results of LES showed that the use of the jet injection from the rear surface of the cylinder greatly reduces the Reynolds stresses while injecting jet from the front surface does not have a significant effect on Reynolds's stresses. Keywords: Square cylinder, active flow control, jet, heat transfer, k-w SST turbulence model, large eddy simulation