We investigate the effect of torsion on DNA condensation with the covalently closed circular DNA as the torsionconstrained system,using an atomic force microscope.It is found that there are two stages in the DNA condensation process under torsional constraint.At the early stage,the low torsion will accelerate DNA condensation by promoting the formation of micro-loops or intersection structures;while at the later stage,the increasing torsion will slow it down by preventing the crosslinking of cisplatin and DNA since the DNA molecule becomes more rigid.Our results show the important role of torsion in DNA condensation and sheds new light on the mechanism for DNA condensation.