Abstract: Hydraulic geometry and tidal volume in tidal estuaries display a relationship of power function and are characterized by fractal dimensions. This paper describes hydraulic geometry of tidal estuaries using the fractal theory and the principle of dimensional homogeneity, along with two in-situ observable indicators: the relative volume of flood or ebb tide for relative cross-sectional areas, and the relative volume of flood or ebb tide per unit width for relative water depths. Formulas have been developed for the relationships of these two indicators versus relative cross-sectional area and relative water depth respectively, both applicable to regular estuaries, but the latter one is also applicable to those open seas of which no cross-section can be defined. In a case study of the Jiaojiang estuary and Oujiang estuary, the parameters in these two formulas of hydraulic geometry were determined using the data of field observation, and the results indicated a fairly good accuracy of the formulas. Fractal dimensions of hydraulic geometry reflect the stability of an estuary and the relative effects of its hydrodynamic conditions during flood tide and ebb tide, and their values will be closer to 1.0 if it is relatively stable or its hydrodynamic forces are stronger. The results would lay a theoretical basis for further studies on estuary evolution and channel design and regulation.