  We propose a Bayesian method to measure the total Galactic extinction
parameters, $R_V$ and $A_V$. Validation tests based on the simulated data
indicate that the method can achieve the accuracy of around 0.01\,mag. We apply
this method to the SDSS BHB stars in the northern Galactic cap and find that
the derived extinctions are highly consistent with those from \cite{SFD98}. It
suggests that the Bayesian method is promising for the extinction estimation,
even the reddening values are close to the observational errors.
