  In the past two years, tremendous progresses to understand X-ray Flashes have
been made. Now it is widely believed that X-ray Flashes and Gamma-ray Bursts
are intrinsically the same, their much different peak energy and flux may be
just due to our different viewing angles to them. Here we analytically
calculate the very early afterglow of X-ray Flashes, i.e., the reverse shock
emission powered by the outflows interacting with the interstellar medium.
Assuming $z\sim 0.3$, we have shown that typically the R Band flux of reverse
shock emission can be bright to $16\sim 17$th magnitude (the actual values are
model dependent and sensitive to the initial Lorentz factor of the viewed
ejecta). That emission is bright enough to be detected by the telescope on work
today such as ROTSE-III or the upcoming UVOT carried by Swift Satellite,
planned for launching in later 2004.
