Abstract: The RayleighTaylor instability caused by a density gradient is very important in the inertial confinement fusion. With the passive scalar transport model, behaviors of the interface with various initial length scales are simulated. It appears that the initial form has a significant influence upon the interfacial motion. Long grooves grow more slowly than square shaped perturbations. And generally the mixing process in the vertical direction may be decreased by the interaction between bubbles with different scales.