We explore two possible models which might give rise to bright X-ray flares in GRBs afterglows. One is an external forward-reverse shock model, in which the shock parameters of forward/reverse shocks are taken to be quite different. The other is a so called late internal shock model, which requires a refreshed unsteady relativistic outflow generated after the prompt $gamma-$ray emission. In the forward-reverse shock model, after the time $t_times$ at which the RS crosses the ejecta, the flux declines more slowly than $(t_oplus/t_times)^{-(2+beta)}$, where $t_oplus$ denotes the observers time and $beta$ is the spectral index of the X-ray emission. In the ``late internal shock model, decaying slopes much steeper than $(t_oplus/t_{rm e, oplus})^{-(2+beta)}$ are possible if the central engine shuts down after $t_{rm e, oplus}$ and the observed variability timescale of the X-ray flare is much shorter than $t_{rm e, oplus}$. The sharp decline of the X-ray flares detected in GRB 011121, XRF 050406, GRB 050502b, and GRB 050730 rules out the external forward-reverse shock model directly and favors the late internal shock model. These X-ray flares could thus hint that the central engine operates again and a new unsteady relativistic outflow is generated just a few minutes after the intrinsic hard burst.
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