Rayleigh-Taylor (RT) instability has a growing importance in many fields including aircraft industry and astrophysics. The development and the growth of RT instability were investigated using sinusoidal disturbances with different wavelength at the interface of two fluids. Numerical simulations were performed by solving Navier-Stokes unsteady equations with the VOF formulation. Results of the 2D simulation are compared with experimental results. It is shown that beyond a critical time, not captured experimentally, the mushroom shape of RT instability turns into a helical path or breaks down into a patchy shape depending on the shape of disturbance. Nonlinear instabilities responsible for such behaviour are apparent when the wavelength exceeds 10 times a length scale introduced here.