The electro-optical characteristics of the Liquid Crystal on Silicon (hereinafter "LCOS") micro-display on vertically alignment (VA) mode were studied depending on various cell gaps. 5 different cell gaps, such as $1.4{mu}m,;1.8{mu}m,;2.1{mu}m,;2.4{mu}m$ and $2.8{mu}m$, were selected. The reflectance-voltage (R-V) characteristics, distributions of reflected light and reflectance were calculated with 3-dimmensional LC code. At the center of cell, the smallest $1.4{mu}m$ cell gap showed the lowest reflectance and the largest $2.8{mu}m$ cell gap showed the highest reflectance due to the surface anchoring effect. In case of $2.1{mu}m$ cell gap, the sum of reflectance overall cell was the highest value. Considering the reflectance and RV curve characteristic, the optimized cell gap was $2.1{mu}m$.