This paper deals with providing a surgical robot with awareness of the current surgical stage. The awareness of the surgical stage is the first step toward a natural interaction between a surgeon and a surgical robot, the ultimate goal of which is to help the surgeon perform surgery with a minimum control burden. For this purpose, a surgery task model was defined as a structured form of surgical knowledge, which can be understood by both the surgeon and the robot. The model consists of three components: a surgical procedure model, input information, and an action strategy at each surgical stage. This paper focuses on the awareness of current surgical stages based on the surgical procedure model. The surgical procedure model represents the sequential information of the surgery and it is arranged based on key surgical stages. To implement the surgical procedure model of a cholecystectomy, 21 cases of human cholecystectomies are decomposed into surgical stages and their relations are then analyzed. To deal with uncertainty, interaction functions are introduced to the model. While further experiments are necessary, it was shown that the key stages-based surgical procedure model could estimate the key surgical stages correctly during one case of in vivo porcine cholecystectomy.