Combination of pressboard and mineral oil continues to be the choice of materials used as the insulation die- lectric media in large transformers. The purpose of the reported study is to investigate the influence of pressboard ag- ing on PD （partial discharge） pulse parameters. Ultra wide band （UWB） time-resolved PD pulse measurements were carried out on gas void inside pressboard. The rise rime, full time and width time of PD pulse in different aging degree pressboard are obtained. The result is helpful to better understanding the influence of the aging of pressboard on PD measurement and insulation diagnostics in power transformers.