Recently the hydroforming technology has drawn a lot of attention because of its capability to produce high quality and light weight parts. In the present study, the tube expansion - one of the simplest hydroforming processes, has been investigated in order to understand fundamental phenomena such as deformation characteristics and effect of process parameters. As a result, the most important process parameters, which determine the state of stress at the expanded zone, were found to be pressure and die displacement. If the stress becomes equi-axial tension at the zone, necking occurs at some distance from the weld line and develops into a crack along the axial direction. Some aspects of mechanical property measurements as well as distributions of hardness and microstructure are also discussed in this paper.