We measure the entropic elasticity of a single λ-DNA molecule and of a single DNA-Psoralen complex by doing stretching experiments with an optical tweezers. Psoralen is a photosensitive drug used in the treatment of many skin diseases, by impeding DNA replication. Psolaren intercalates the DNA and can form crosslinks with pyrimidine basis in opposite strands of DNA, when illuminated with UVA light. As crosslinks form the persistence length of the complex increases, indicating an increase in rigidity of the complex. We study the kinetics of DNA-Psoralen crosslink formation via changes in entropic elasticity of the complex.