This paper presents an approach to developing Real-time Dependable Systems (RDSs) based on a real-time object model. A system development process can be divided into two phases, that is, Specification Phase and Construction Phase. In the Specification Phase, especially for an RDS, system requirements are decomposed into a group of Real-Time Objects (RTOs) and appropriate objects are identified by means of a decomposition scheme such as Multi-Dimensional Decomposition (MDD). In the Construction Phase, the identified objects are instantiated and combined. RTO.e, which is an extension of the RTO.k model proposed earlier by Kim and Kopetz, plays the role of a connector and also that of a firewall between the two phases. In this paper, we present design tools which are required in developing RDSs. The tools provide a seamless system implementation method and an information base which can be utilized through the development process.