We solve the longstanding Gaussian curvature conjecture of a minimal graph S over the unit disk. It states the following. For any minimal graph lying above the entire unit disk, the Gauss curvature at the point above the origin satisfies the sharp inequality |K| < π 2 /2. The conjecture is firstly reduced to the estimating the Gaussian curvature of certain Scherk's type minimal surfaces over some bicentric quadrilaterals inscribed in the unit disk containing the origin inside. Then we make a sharp estimate of the Gaussian curvature of those minimal surfaces over those bicentric quadrilaterals at the point above the zero.