Motivated by the isometric embedding problem and fully nonlinear PDE theory, we study the apriori estimate for scalar curvature equations. Joint with Prof. Pengfei Guan, we proved that there is an interior second order estimate for isometrically embedded hypersurfaces with positive scalar curvature. By employing Warren and Yuan's integral method and my new observation in three-dimensional hypersurface with positive scalar curvature, I give an affirmatively answer to the interior second order estimate to this fully nonlinear PDE in dimension three.