This paper considers the length-lex domain for set-variables in constraint programming which includes not only membership but also cardinality and lexicographic information. The paper studies how to enforce bound consistency on a knapsack constraint over a set variable in this domain and proposes a bound-consistency algorithm which runs in O(c log n) time with a one-time preprocessing cost O(cn2 ) when the constraint is posted. Categories and Subject Descriptors D.3.2 [Language Classifications]: Constraint and logic languages; D.3.3 [Language Constructs and Features]: Constraints Keywords Set variable, Length-lex, Bound Consistency, Knapsack