In　order　to　diagnosis　the　gas　fluctuations　of　aerostatic　guideways,　the　calculation　equation　of　stiffness　of　aerostatic　guideway　with　double　row　orifice　has　been　derived　by　gas　lubrication　theory.　For　the　stiffness　coupling　structure　of　guideway　and　unloading　guideway,　the　frequency　of　whole　slides　in　machining　direction　with　gas　thickness　has　been　derived　by　dynamic　model.　Wavelet　transform　as　a　means　to　process　the　measured　signal　of　workpeice　surface,　the　frequency　characteristics　of　gas　fluctuation　of　aerostatic　guideways　has　been　extracted　out　from　measured　signal,　the　errors　of　guideway　system　have　been　diagnosed　according　to　power　spectral　density　(PSD),　this　provide　a　basis　for　the　diagnosis　of　machine　tool　error.　(C)　2010　Elsevier　Ltd.　All　rights　reserved.