t A numerical simulation and study of the transition to chaos of the flow past a two-dimensional circular cylinder is presented.Using tools from dynamical system theory,such as phase space diagrans,Frequency analysis,Poincare seetions and the calculation of statistical quantities of chaos:thge correlation dimension,Kolmogorov entropy,Laypunov exponent and so on,the different solutions of Navier-Stokes equation at different values of Reynolds unmber have been identifled and characterized through rreconstructing the phase space of velocity time series by means of time delay and dimension cmbeding.It signifles that the solutions are periodic orr approximatelv periodic in some speeifled range of Reynolds unmber and that a very cimpplex transition occurs with bifureation route to chaos.