The Unified Modeling Language (UML) has been designed to be a full standard notation for Object Oriented Modeling. UML has nine types of diagrams [3, 6]: class, object, sequence, collaboration, use case, statechart, activity, deployment and, component diagram. All these diagrams together are used to describe one model. There are some basic rules should be followed when drawing a diagram. Also, one diagram could be correct but inconsistence with another. This paper proposed fifteen basic rules that help developers to build correct and consistent UML diagrams.