We apply density functional theory (DFT) and evolutionary structure prediction algorithms to study the high-pressure behavior of sulfur above 100 GPa. Based on DFT calculations, we determined the energetic relations between experimentally observed S-III, S-V, and β-Po structures. S-V is found to be unstable and transforms to S-III under optimization. The second order character of transformation from S-III to β-Po is theoretically justified and calculations yield a pressure of about 250 GPa. β-Po structure is not stable in trigonal symmetry below 250 GPa and spontaneously transforms into a monoclinic structure identical to S-III. The calculations yield pressures of 333 and 460 GPa for the transitions from β-Po to simple cubic (sc) and from simple cubic to body centered cubic (bcc) structures of sulfur, respectively.