Abstract: Service innovation is the basis for the survival of professional service sectors. However, as the innovative service is often imitable, professional service firms are facing severe challenges of imitation, especially under weak appropriability regime in China. To explore the appropriability mechanisms in professional service sector, this paper conducts exploratory case study and draws three conclusions based on the grounded theory: 1) appropriability mechanism in professional service sector is defined as the means for professional service firms to prevent imitation, protect both innovation itself and the increased rents due to innovation activities. Three endogenous mechanisms are property protection, internal isolation and customer anchored, constituting two different appropriability strategies, as the former two for defensive strategy and the latter one for aggressive strategy. Besides, the aggressive strategy is more emphasized. 2) Appropriability mechanisms are more focused on increasing appropriability revenue rather than reducing imitation. 3) The three appropriability mechanisms have different positive effects on reducing imitation and increasing appropriability revenue.