In view of the importance of one-dimensional (1D) thermal-hydraulic analysis for rod bundle geometry, extensive efforts to improve constitutive equations have been made in recent years. In the present article, the state-of-the-art rod-bundle constitutive equations for flow regime map, void fraction, covariance, and interfacial area concentration models are reviewed. Among them, the constitutive relations for covariance and interfacial area concentration models may improve the conventional analysis method’s robustness. Some sensitivity analysis results using TRAC-BF1 code with the new constitutive equations are summarized and reviewed.