Variability is one of the hallmarks of Active Galactic Nuclei .
The Burst Alert Telescope onboard of Swift , with its homogeneous coverage of the sky is a formidable tool to study variability at hard X-rays .
We present here the analysis of the 1-month binned Swift /BAT lightcurves of the 20 brightest Active Galactic Nuclei in the hard X-ray sky .
The sample consists of 2 blazars , 3 radio galaxies , 6 Seyfert 1/1.5s , 8 Seyfert 2s and 1 Narrow Line Seyfert 1 .
We found that all the objects show variability , and most of them have a value of the fractional root mean squared variability amplitude of F _ { var } \sim 0.2 - 0.3 .
We did not find any significant correlation of F _ { var } with the column density or the luminosity in our sample .