Moisture　in　power　transformer　strongly　influences　the　dielectric　properties　of　oil-paper　insulation　system.　The　insulation　paper　gradually　degrades　over　time,　therefore,　it　is　imperative　to　assess　integral　condition　of　oil-paper　insulation　for　assuring　a　normal　operation　of　power　transformers.　As　a　non-destructive　diagnostic　technique,　return　voltage　measurement　is　an　effective　assessment　method　of　water　content　in　transformer　pressboard.　The　study　presented　in　this　paper　aims　of　further　investigating　the　reliability　of　the　quantification　of　water　content　using　dominant　time　constant　in　transformer　pressboard.　Field　tests　performed　on　several　transformers　in　different　status　have　shown　that　the　return　voltage　measurement　method　can　estimate　the　moisture　content　in　paper　insulation　of　power　transformers　effectively.　©2010　IEEE.