  We introduce a new method to determine the redshift of unknown-redshift BL
Lac Objects. The method relies on simultaneous multi-wavelength (MWL)
observations of BL Lac objects in optical, X-ray, HE (E>100 MeV) gamma-rays and
VHE (E>100 GeV)gamma-rays. It involves best-fitting spectral energy
distribution (SED) from optical through HE gamma-rays with a
Synchrotron-Self-Compton (SSC) model. We extrapolate such best fitting model
into VHE regime, and assume that it represents the intrinsic emission of the
object. We then compare the observed VHE flux which has been affected by the
interaction with Extragalactic Background Light (EBL). Constraining the
measured vs intrinsic emission leads to the determination of gamma-gamma
opacity. Comparing the obtained opacity with the predicted opacity based on EBL
model, we obtain the redshift of the photon source.
