A context aware system and a context aware method thereof perform a reasoning to be aware of contexts in ubiquitous environments. The context aware method includes procedures of modeling a knowledge base describing context information including constraints; classifying the context information described in the knowledge base according to the constraints; filtering fact data having same constraints as input sensor data from the classified context data; storing only rules, which satisfy the filtered fact data and constraints, in an working memory; and inferring a service context by reasoning the stored fact data and rules. The method receives input sensor data, associated with a subject, continuously monitored and serviced by indoor sensors, performs a reasoning based upon the input sensor data, in order to be aware of the present contexts of the subject and also provide, in real-time, a service suitable for the present contexts, thereby promoting the convenience of life.