  So far essentially all black hole masses in X-ray binaries have been obtained
by observing the companion star's velocity and light curves as functions of the
orbital phase. However a major uncertainty is the estimate of the orbital
inclination angle of an X-ray binary. Here we suggest to measure the black hole
mass in an X-ray binary by measuring directly the black hole's orbital motion,
thus obtaining the companion to black hole mass ratio. In this method we assume
that accretion disk wind moves with the black hole and thus the black hole's
orbital motion can be obtained from the Doppler velocity of the absorption
lines produced in the accretion disk wind. We validate this method by analyzing
the Chandra/HETG observations of GRO J1655-40, in which the black hole orbital
motion with line of sight velocity of 90.8 (+-11.3) km/s, inferred from the
Doppler velocity of disk-wind absorption lines, is consistent with the
prediction from its previously measured system parameters. We obtain the black
hole mass of 5.41 (+0.98, -0.57) solar masses and system inclination of 72.0
(+7.8, -7.5) degrees in GRO J1655-40. Additional observations of this source
covering more orbital phases can improve estimates on its system parameters
substantially. We then apply the method to the black hole X-ray binary LMC X-3
observed with HST/COS near orbital phase 0.75. We find that the disk-wind
absorption lines of CIV doublet were shifted to about 50 km/s, which yields a
companion-to-black-hole mass ratio of 0.6 for an assumed disk wind velocity of
-400 km/s. Additional observations covering other orbital phases (0.25 in
particular) are crucial to ease this assumption and then to directly constrain
the mass ratio. This method in principle can also be applied to any accreting
compact objects with detectable accretion disk wind absorption line features.
