In this study a phase diagram has been used to investigate the unsteadiness of two-dimensional lid-driven closed flows within a square cavity for twelve Reynolds numbers; $7.5{ imes}10^3,; 8{ imes}10^3,; 8.5{ imes}10^3,; 9{ imes}10^3,; 9.5{ imes}10^3,; 10^4,;1.5{ imes}10^4,;2{ imes}10^4,; 3{ imes}10^4,; 7.5{ imes}10^4$ and $10^5.$ The results indicate that the first critical Reynolds number at which the flow unsteadiness of sinusoidal fluctuation appears from the temporal variation of total kinetic energy curves is assumed of sinusoidal fluctuation appears form the temporal variation of total kinetic energy curves is assumed to be in the neigh-bourhood of $Re=8.5{ imes}10^3$ The second critical Reynolds number where the periodic amplitude and frequency collapse to random disturbance being existed around $Re=1.5{ imes}10^4$ The exponentially decreasing vortices formed at the lower two corners are found commonly at the time-mean flow pattern of $Re=3{ imes}10^4.$