In order to investigate the effect of pressure on the tertiary structure of plasmid DNA having the supercoiled and relaxed forms, the solution of plasmid DNA was hydrostatically pressurized at different atmosphere and 40 degrees C for various times. For dynamic light scattering (DLS) measurement of the pressurized plasmid DNA, the hydrodynamic diameters of the super-coiled and relaxed plasmid DNA were decreased with increasing pressure. Also, at constant pressure, a long period of pressure treatment effectively induced the decrease in plasmid DNA. These results suggest that the plasmid DNA was condensed by high hydrostatic pressurization. The circular dichroism (CD) spectrum of the pressurized plasmid DNA was slightly changed. For digestion by S1 nuclease, which selectively cleaves single strand DNA, the pressurized plasmid DNA was easily degraded compared to the non-pressurized plasmid DNA, suggesting that the double helix of plasmid DNA was partly dissociated to single strand by the pressure-induced compaction of plasmid DNA. These results indicate that high hydrostatic pressurization is one of powerful tools for preparing the compacted plasmid DNA.