A new ontology-based question expansion ( OBQE ) method is proposed for question similarity calculation in a frequently asked question (FAQ) answering system. Traditional question similarity calculation methods use “word” to compose question vector, that the semantic relations between words are ignored. OBQE takes the relation as an important part. The process of the new system is: ①) to build two-layered domain ontology referring to WordNet and domain corpse; ② to expand question trunks into domain cases; ③ to use domain case composed vector to calculate question similarity. The experimental result shows that the performance of question similarity calculation with OBQE is being improved.