In the application of fuzzy reasoning, researchers usually choose the membership function optionally in some degree. Even though the membership functions may be different for the same concept, they can generally get the same or approximate results. The robustness of the membership function optionally chosen has brought many researchers' attention. At present, many researchers pay attention to the structural interpretation definition of a fuzzy concept, and find that a hierarchical quotient space structure may be a better tool than a fuzzy set for characterizing the essential of fuzzy concept in some degree. In this paper, first the uncertainty of a hierarchical quotient space structure is defined, the information entropy sequence of a hierarchical quotient space structure is proposed, the concept of isomorphism between two hierarchical quotient space structures is defined, and the sufficient condition of isomorphism between two hierarchical quotient space structures is discovered and proved also. Then, the relationships among information entropy sequence, hierarchical quotient space structure, fuzzy equivalence relation, and fuzzy similarity relation are analyzed. Finally, a fast method for constructing a hierarchical quotient space structure is presented.2 for all x, y ∈ X, R x, y R y, x , then R is called a fuzzy similarity relation on X. Definition 2.2 see 6 . Let R be a fuzzy relation on X. If it satisfies, 1 for all x ∈ X, R x, x 1, 2 for all x, y ∈ X, R x, y R y, x , 3 for all x, y, z ∈ X, R x, z ≥ sup y∈X min R x, y , R y, z , the R is called a fuzzy equivalence relation on X and is denoted by R. Proposition 2.3 see 17 . Let R be a fuzzy similarity relation on X, and let R denote the transitive closure of R. Then, for all m ≥ n, R R m .According to Proposition 2.3, a fuzzy equivalence relation can be induced from a fuzzy similarity relation by R → R 2 → R 2 2 → · · · → R 2 k R. Where k ≥ log 2 n. Proposition 2.4 see 17 . Let R be a fuzzy equivalence relation on X, and R λ { x, y | R x, y ≥ λ} 0 ≤ λ ≤ 1 , then R λ is a crisp equivalence relation on X, R λ is called cut-relation of R.