Abstract: This paper proposes a unified reduced element applicable to both thick and thin beams based on Timoshenko-and-Euler beam theories. By using the energy variational approach, the built-in maximum error estimator is constructed and the corresponding adaptive analysis algorithm is developed. The proposed element inherits the advantages of reduced elements in other applications, and can provide, for both thick and thin beams, desired reduced element solutions that satisfy the user-preset error tolerances by the maximum norm. A series of numerical examples are given to show the feasibility, effectiveness and reliability of the proposed method.