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Which version of PlantUML are you using ? The following example seems to work fine for us: @startuml skinparam backgroundColor #AAAAAA Bob -> Alice : hello @enduml http://plantuml.com/plantuml/svg/AyxEp2j8B4hCLKXAJCvEByelpKjnpi_9Br9GTWG3BgVy90LTEmN7dCpaL0KhXOpKd9ny0000 In the SVG source, you will find the following attribute: style="width:119px;height:125px;background:#AAAAAA;"