Context-based access control is an emerging approach for modeling adaptive solution, making access control management more flexible and powerful. But in the ubiquitous environment, this approach is not enough for many emerging security vulnerabilities. Thus, improving current access control mechanisms is still necessary. Risk is an effective tool used for decision-making in economics. In this paper, we design a new model for risk assessment in ubiquitous environment and use risk as a key component in decision-making process in our access control model. This solution makes access control management more dynamic and precise.