The interactions of the several linear DNA fragments and cisplatin at different concentrations and incubation time have been studied by atomic force microscopy. When equal volume of 2mM cisplatin and 5ng/microl 1134bp DNA fragment were incubated for 30min, 1h, 2h and 3h, the observed DNA were gradually changed from the extended chains to the bending structures, the worm-like structures and flat particles. The formation of these structures can be attributed to the coordination and electrostatic interactions by binding of cisplatin on DNA. When longer DNA fragments such as pBR322/pstI DNA (4361bp) and lambda DNA (48502bp) were used for interacting with cisplatin in the same condition, the observed DNA molecules tended to form many loop structures. The differences of the interactions between long and short DNA can be ascribed to the strong intramolecular interactions within the long DNA fragment. When the relative low concentration of cisplatin (100micromol/l) was used in the experiments, the observed DNA tended to bend along its chains. Furthermore, the measured contour length and the calculated persistent length of cisplatin-DNA were gradually decreased with the incubated time. All of these AFM results revealed that the cisplatin-DNA interactions in vitro can be influenced by the concentration and the initial state of cisplatin, the length of DNA fragment and the reaction time.