A crucial question in accelerating a foil to relativistic velocity is its hydrodynamic stability. In particular the relativistic Rayleigh - Taylor (RT) instability was calculated (Pegoraro and Bulanov, 2007) as described in Equation (2). xNR and xR are accordingly the non-relativistic and relativistic development of the instability for an initial disturbance x0 and lT is the target dimension orthogonal to the x amplitude. For example, if lT ¼ 10 mm, x0 ¼ 10 nm, r0 ¼ 1 g/cm3, l ¼ 0.1 mm, and assuming that the foil breaks for x ¼ 10 (i.e. Dx w l) then I ¼ 1024 W/cm2 will cause the foil to break at 2.5fs for the non-relativistic case while in the relativistic regime the foil is stable 50fs. This behavior is understood from the different time dependence of the RT instability in relativistic and nonrelativistic cases. Therefore we conclude that for the relativistic acceleration by ultra-intense laser the foil is stable under RT instability.