Abstract: In order to enhance the support for uncertain knowledge representation and reasoning, an ontology modeling approach based on certainty factor was proposed, by combining the certainty factor with ontology model. An uncertain reasoning method based on certainty factor modification was given, which modifies the certainty factors of knowledge according to evidence reliability, and makes the reasoning results more consistent with the real world by improving the synthesization of conclusion uncertainty. Case study about a home health telemonitoring application shows that the proposed approach is feasible and efficient.