Asymmetric horizontal thermal profiles can change the flow field structure in the physical vapor transport (PVT) of Hg2Cl2. It is found that for the ratios of horizontal to vertical thermal Rayleigh numbers Ra-H/Ra greater than or equal to 1.5, the convetive flow structure changes from multicellular to unicellular for the base parametric state of Ra = 2.79 x 10(4), Pr = 0.91, Le = 1.01, Pe = 4.60, Ar = 0.2 and C-s = 1.01. The unicellular flow structure obtained by increasing Delta T-H*(Ra-H) is not likely to support the conjecture of the presence of unicellular convective mode in the laser Doppler velocimetry (LDV) experiments for PVT process of Hg2Cl2 (Kim et al., 1996). When the six parameters of Ra, PI, Le, Pe, Ar and C, are fixed, the dimensional maximum magnitude of the velocity vector \U\(max) is proportional to root Delta T-H*.