The effect of Mg2+ on hydrothermal formation of alpha-CaSO4 center dot 0.5H(2)O whiskers with high aspect ratios was investigated in this paper. alpha-CaSO4 center dot 0.5H(2)O whiskers with a preferential growth along the c axis and an average aspect ratio up to 370 were synthesized using hydrothermal treatment of CaSO4 center dot 2H(2)O precursor in the presence of 1.97 X 10(-3) mol.L-1 MgCl2. The preferential adsorption of Mg2+ on the negative (200), (400), and (020) facets was confirmed by EDS, XPS, and zeta potential measurements. ATR-FTIR analysis revealed the ligand adsorption of Mg2+ on the surface of alpha-CaSO4 center dot 0.5H(2)O. The doping of Mg2+ in alpha-CaSO4 center dot 0.5H(2)O whiskers was confirmed by the XRD analysis. The experimental results indicated that the adsorption and doping of Mg2+ promoted the 1-D growth of alpha-CaSO4 center dot 0.5H(2)O whiskers, leading to the formation of whiskers with high aspect ratios.