The direct simulation Monte Carlo (DSMC) method is used to simulate micro backward facing step flows in both slip and transition flow regimes. A DSMC code was developed and validated with micro channel flows. The effects of Kn number in flow characteristics are analyzed in detail. It is found that the phenomena of flow separation, recirculation, and reattachment will disappear as Kn number exceeds 0.1. A significant jump of pressure and velocity behind the step is observed in the transition flow regime. The effect of the compressibility is also discussed. The compressibility has significant effect on flo characteristics in the slip flow regime but would be negated by the rarefaction in the transition flow regime.