LEED patterns are obtained by calculation using Fourier graphical method for Si(100) double stepped model surface. Five model surfaces give different patterns one another though each model has only a little change in the structure. This result could be used in the experimental characterization of double stepped Si(100) surface structure. Si(100) single stepped surface is studied by LEED experiment. Depending on sample preparation conditions (2x1), (2x2), (2x8) and half-order streak pattern appeared. High temperature annealing followed by radiation quenching gives (2x2) and (2x8) reconstruction pattern, while lower temperature annealing and slow cooling give half-order streaks respectively. LEED pattern is independent of Ar ion dose bombarded to sample and annealing time within the our LEED resolution limits. We propose two model surface corresponding to high temperature annealing followed by quenched and low temperature annealed respectively.