In this paper, a new agent development platform, which includes built-in features for semantic web based multi agent system development, is introduced. All agents and services in the platform use semantic web standards to represent their internal knowledge and semantic web query languages are used to query them. Semantic web services can be discovered and dynamically invoked. The directory service is implemented in a way to support semantic matching of agent capabilities. The ontology service allows ontology translation based on defined mappings between platform ontologies and external ontologies. With these features already integrated, the agent development platform that is developed simplifies the semantic web based multi-agent system development. Categories and Subject Descriptors H.3.3 [Information Storage and Retrieval]: Information Search and Retrieval General Terms Design