Merger rate density of BH-BH mergers from Population I/II stars. We employ models M10.B and M30.B to illustrate the effects of our assumptions on the star formation rate and cosmic metallicity evolution on the BH-BH mergers. LIGO/Virgo O1/O2 constraint on the BH-BH merger rate in local Universe is also shown. We note that the decrease of the BH-BH merger rate density at low-redshift (from old to new models) is due to the average metallicity of stars at any given redshift in old models, as it is much lower than the average metallicity of stars in new models (see Sect. 3.2 for details).