  Interstellar extinction in ultraviolet is the most severe in comparison with
optical and infrared wavebands and a precise determination plays an important
role in correctly recovering the ultraviolet brightness and colors of objects.
By finding the observed bluest colors at given effective temperature and
metallicity range of dwarf stars, stellar intrinsic colors, $C^0_{\rm B,V}$,
$C^0_{\rm NUV,B}$, $C^0_{\rm FUV,B}$ and $C^0_{\rm FUV,NUV}$, are derived
according to the stellar parameters from the LAMOST spectroscopic survey and
photometric results from the $GALEX$ and APASS surveys. With the derived
intrinsic colors, the ultraviolet color excesses are calculated for about
25,000 A- and F-type dwarf stars. Analysis of the color excess ratios yields
the extinction law related to the $GALEX$ UV bands: $E_{{\rm NUV,B}}$/$E_{{\rm
B,V}} = 3.77$, $E_{{\rm FUV,B}}$/$E_{{\rm B,V}} = 3.39$, $E_{{\rm
FUV,NUV}}$/$E_{{\rm B,V}} = -0.38$. The results agree very well with previous
works in the $NUV$ band and in general with the extinction curve derived by
Fitzpatrick (1999) for $R_{\rm V}=3.35$.
