The　spectral　leakage　has　great　influence　on　the　performance　of　dielectric　loss　angle　measurement　when　using　fast　Fourier　transform　(FFT)　algorithm.　Based　on　the　analysis　of　the　spectrum　leakage　effect　of　FFT　algorithm,　an　improved　FFT　method　combined　Nuttall　window　with　five-point　polynomial　transform　to　detect　dielectric　loss　angle　was　presented　in　this　paper.　Since　Nuttall　window　has　a　good　attenuation　characteristic　of　side　lobe,　and　five-point　polynomial　transform　can　speed　up　the　attenuation　of　side　lobe　spectrum,　the　proposed　method　firstly　applied　Nuttall　window　to　truncate　the　measurement　signal,　then　used　FFT　weighted　polynomial　transformation　so　as　to　reduce　the　output　sequence　spectrum　leakage　effect　and　improve　the　analysis　accuracy.　Through　simulations　and　experiments,　the　proposed　Nuttall　window　and　five-point　polynomial　transform　interpolated　FFT　algorithm　is　proven　to　be　accurate　and　reliable.　The　impact　factors　of　dielectric　loss　angle　measurement　including　fundamental　frequency　fluctuation,　number　of　sampling　points,　random　noise,　harmonic　component,　and　direct　current　component　are　also　analyzed　in　detail　in　this　paper.　©　2015,　The　editorial　office　of　Transaction　of　China　Electrotechnical　Society.　All　right　reserved.