Flat3D is a shared 3D virtual world system that can be grown by user?s creative activities and communication over the network. In conventional shared virtual 3D world systems, users sharing the virtual 3D world communicated with each other only by text-based chatting through avatars, so the communication capabilities were quite limited. In contrast, the Flat3D system allows users to create objects freely in a wade 3D world by simple manipulations. These creative activities become communication trigger and scene elements directly, so the virtual 3D world would be grown by user?s creation and communication. In this paper, we describe the Flat3D system and its capabilities.