Evolution of first population of massive metal-free binary stars is followed .
Due to the low metallicity , the stars are allowed to form with large initial masses and to evolve without significant mass loss .
Evolution at zero metallicity , therefore , may lead to the formation of massive remnants .
In particular , black holes of intermediate-mass ( \sim 100 - 500 M _ { \odot } ) are expected to have formed in early Universe , in contrast to the much lower mass stellar black holes ( \sim 10 M _ { \odot } ) being formed at present .
Following a natural assumption , that some of these Population III stars have formed in binaries , the physical properties of first stellar binary black holes are presented .
We find that a significant fraction of such binary black holes coalesces within the Hubble time .
We point out that burst of gravitational waves from the final coalescences and the following ringdown of these binary black hole mergers can be observed in the interferometric detectors .
We estimate that advanced LIGO detection rate of such mergers is at least several events per year with high signal to noise ratio ( \gtrsim 10 ) .