The data and information from heterogeneous embedded devices lead to security and communication problems in M2M environment. Even under the situation where the low-level information causes interoperability problems, these devices should be able to communicate with each other in safe, efficient and right manner without the help of server or human intervention. In this paper, we propose a security based context aware U-healthcare system based on user authentication and data aggregation. Our system can support medical decision-making and inferences using Knowledge Machine (KM), which is a mature knowledge representation language. We design and implement frame-based ontology and rules for semantic reasoning using LISP+KM combination. We especially describe the context query, and then show the progress made towards proper and reasonable inference and prediction. In experiments, we showed the feasibility of our proposed system by adding intentional abnormal/informal situation data.