Rule based systems and agents are important applications of the Semantic Web constructs such as RDF, OWL, and SWRL. While there are plenty of utilities that support ontology generation and utilization, rule acquisition is still a bottleneck as an obstacle to wide propagation of rule based systems. To automatically acquire rules from unstructured texts, we develop a rule acquisition framework that uses a rule ontology. The ontology can be acquired from the rule base of a similar site, and then is used for rule acquisition in the other sites of the same domain. The procedure of ontology-based rule acquisition consists of rule component identification and rule composition. The former uses stemming and semantic similarity to extract variables and values from the Web page and the latter uses the best-first search method in composing the variables and values into rules.