This paper covers modeling and identification of one joint of an industrial robot manipulator including flexibilities. It is shown how models can be built in the Modelica graphical environment and how these models can be transformed into a mathematical state space description which directly can be used for identification. A motivation to use linear models for modeling of the robot arm is given. This includes an analysis of the nonlinearities in the input-output data from the actual robot using a particular kind of input signal. Identification and validation of the physically parameterized models are also covered.