An a accurate evaluation of the growth rate of ablative Rayleigh-Taylor (RT
) instability is a critical issue for inertial confinement fusion (ICF). Wh
en the preheat is considered,the growth rates of ablative RT instability by
two-dimensional (2D) simulations are lower and agree with those of the exp
eriment at Lawrence Livermore National Laboratory (LLNL). In this case, big
ger difference occurs between the Lindl formula gamma = root kg/(1 + kL) -
beta kV(a) and 2D simulations. The lower Atwood stabilization due to the pr
eheat is observed in simulations. A new growth rate formula gamma = root Ak
g/(1 + AkL) - 2kV(a) is obtained and agrees well with all of 2D simulations
. It includes the density gradient stabilization,the lower Atwood stabiliza
tion and the ablative convection stabilization. It reduces to the Lindl for
mula when A = 1 and is close to the Takabe formula when the ablation front
is sharp.