Although the well-known master curve method has been widely used to estimate the ductile to brittle transition temperature and to prevent subsequent brittle fracture of reactor pressure vessels, it has a limitation to determine reference temperatures on heat affected zone and weld metals. The present paper is to propose an enhanced master curve method. Prior to this, several schemes to provide the master curve of inhomogeneous materials such as bimodal master curve estimation, randomly inhomogeneous master curve estimation and single point estimation were reviewed to confirm their applicability. As a result, the single point estimation scheme was chosen as the basic algorithm to calculate the reference temperature and modified by adopting three $T_{0sp}$ parameters concept to enhance the accuracy. The proposed method can be used to inhomogeneous materials for more accurate reference temperature calculation of reactor pressure vessels even with dispersed fracture toughness test data.