  It is well established that timing and spectral properties of Galactic Black
Hole (BH) X-ray binaries (XRB) are strongly correlated. In particular, it has
been shown that low frequency Quasi-Periodic Oscillation (QPO) frequency -
photon index correlation curves have a specific pattern. In a number of sources
the shape of the index-low frequency QPO correlations are self-similar with a
position offset in the QPO frequency-Gamma plane. Titarchuk & Fiorito presented
strong theoretical and observational arguments that the QPO frequency values in
this QPO frequency-Gamma correlation should be inversely proportional to
M_{BH}. A simple translation of the correlation for a given source along the
frequency axis leads to the observed correlation for another source. As a
result of this translation one can obtain a scaling factor which is simply a BH
mass ratio for these particular sources. This property of the correlations
offers a fundamentally new method for BH mass determination in XRBs. Here we
use the observed QPO-index correlations observed in three BH sources: GRO
J1655-40, GRS 1915+105 and Cyg X-1. The BH mass of (6.3 +/- 0.5) M_Sun in GRO
J1655-40 is obtained using optical observations. RXTE observations during the
recent 2005 outburst yielded sufficient data to establish the correlation
pattern during both rise and decay of the event. We use GRO J1655-40 as a
standard reference source to measure the BH mass in Cyg X-1. We also revisit
the GRS 1915+105 data as a further test of our scaling method. We infer the
value of BH mass of (15.6 +/- 1.5) M_Sun in this source which is consistent
with the previous BH mass estimate in GRS 1915 of (13.3 +/- 4) M_Sun. We obtain
the BH mass in Cyg X-1 in the range (8.7 +/- 0.8) M_Sun.
