A theory of branch parameter estimation based on dominance assessment is presented. First， to raise the measurement redundancy in parameter estimation， the definitions of dominant parameters and non-dominant parameters are given based on the application target of parameter estimation and the effect branch parameters have on state estimation， and the dominant parameters are selected as the parameters to be estimated. Second， to avoid the interference of the parameters with large diversity in the dominance， the definitions of strong dominant parameter and weak dominant parameter are further proposed， and an iterative estimation method is presented. The flow chart of branch parameter estimation based on dominance assessment and the implementation technique to determine dominant parameters are also given. Finally， the effectiveness of the new theory is tested on the IEEE 14-bus system and the IEEE 300-bus system.