The viscous, incompressible and laminar flow around rectangular cross sections is simulated via a Cartesian-Staggered grid finite volume method for various of aspect ratios. Simulations were carried out for a range of 10–150 Reynolds number and aspect ratios of 0.5–4. In the present work, The Great-Source Technique is used to satisfy directly the no slip condition. The grid is refined in the same size in the back and front regions of the body, but for increasing computation accuracy, the region of rectangular section has been refined more. Because of widespread application of rectangular cross sections and less attendance on it than circular sections, especially in laminar flow and low Reynolds, the aim of this study is to investigate the range of harmonic instability and estimation of its critical Reynolds number. For this purpose, The shedding frequency and The variation of lift and drag coefficients with time and Reynolds numbers have been analyzed for various of aspect ratios. The results show that increase in aspect ratios leads the rise in critical Reynolds number.