As a new type of electroactive polymer (EAP), dielectric elastomers can work in both actuator and generator modes. Dielectric elastomers (DE) can achieve good performance in a generator mode, such as high energy density, good impedance matching to many energy sources, low cost, etc. Based on the structure of dielectric elastomers film, the fundamentals of dielectric elastomers, namely fundamentals of alterable capacitance are discussed. The theoretical models of dielectric elastomers film were analyzed, and the conclusion that Maxwell stress effects could be treated as a piezoelectric material with an effective piezoelectric coefficient was drawn. Power energy conversion cycle was analyzed by simulation and experiments, and the experimental curves of output voltages were investigated. The relation between electric field and elastic restoring stress, and its influence on power generating efficiency, were discussed. At last, the mechanical cycle of power generation for dielectric elastomers was determined approximately.