In this paper, the problem of calibrating a paper machine dryer section model to measurement data is treated. The model, which is dynamic, nonlinear and of large scale, is implemented in Modelica and Dymola. The purpose of the model is to evaluate, in simulation, new control structures before application to the plant. This approach requires a well calibrated model in order for simulation results to be valid also on the real plant. In this paper, the problem is addressed by parameter optimization. A custom application integrating code generated by Dymola and packages for numerical optimization has been developed. Results are satisfactory, in that the mismatch between the calibrated model response and measured data is small.