  The aim of this study is to search for observational evidence of vertical
iron stratification in the atmosphere of fourteen blue horizontal-branch (BHB)
stars. We have found from our numerical simulations that five BHB stars: B22,
B186 in the globular cluster NGC 288, WF2-820, WF2-2692 in M13 and B203 in M15
show clear signatures of the vertical stratification of iron whose abundance
increases toward the lower atmosphere. Two other BHB stars (B334 in M15 and
B176 in M92) also show possible iron stratification in their atmosphere. A
dependence of the slope of iron stratification on the effective temperature was
also discovered. It is found that the vertical stratification of iron is
strongest in BHB stars with Teff around 11,500K. The slope of iron abundance
decreases as Teff increases and becomes negligible for the BHB stars with Teff=
14,000K. These results support the hypothesis regarding the efficiency of
atomic diffusion in the stellar atmospheres of BHB stars with Teff > 11,500K.
