Abstract: With the development of single molecule manipulation, the properties of single biological macro-molecules can be tested by applying a force or torque to it. Mechanical experiments suggest that both force and energy are equally important structural and functional elements for the understanding of bio-chemical processes in biological macro-molecules. A ladder model where the double strand DNA consists of a number of base pairs (parallel rungs of the ladder with attractions between them) linking to two worm-like nucleotide chains (two long sides of the ladder) is presented to describe the force-extension curve of DNA. The theoretical curve obtained by the path integral method agrees well with experimental results. The generated function method in the hybridized statistics theory of polymer chains is used to calculate the elastic behavior of single strand DNA, and the theoretical results are consistent with the force-induced unzipping phase transition observed in experiments. Moreover, it is found that the elastic modulus for DNA sequences determined by p53 (the tumor-suppressor protein) is about one-third of the value for random-sequence DNA.