  The frequency ratio of the 1S0(F=1/2)-3P0(F=1/2) clock transition in 171Yb
and the 1S0(F=9/2)-3P0(F=9/2) clock transition in 87Sr is measured by an
optical-optical direct frequency link between two optical lattice clocks. We
determined the ratio (\nu_{Yb}/\nu_{Sr}) to be 1.207 507 039 343 340 4(18) with
a fractional uncertainty of 1.5x10^{-15}. The measurement uncertainty of the
frequency ratio is smaller than that obtained from absolute frequency
measurements using the International Atomic Time (TAI) link. The measured ratio
agrees well with that derived from the absolute frequency measurement results
obtained at NIST and JILA, Boulder, CO using their Cs-fountain clock. Our
measurement enables the first international comparison of the frequency ratios
of optical clocks, and we obtained a good agreement between the two measured
ratios with an uncertainty smaller than the TAI link. The measured frequency
ratio will be reported to the International Committee for Weights and Measures
for a discussion related to the redefinition of the second.
