Accuracy of sliding mode observers with respect to the errors in internal model parameters is analyzed in the frequency domain. It was shown previously that the SM observer could be analyzed as a relay feedback-feedforward system. It is feedback with respect to the measured variable of the system being observed, and the feedforward with respect to the control applied to the system being observed. This approach is now used for analysis of SM dynamics with internal model having a mismatch with respect to the model of the system being observed. The input-output properties of observer dynamics are analyzed with the use of the Locus of a Perturbed Relay System method. An example of accuracy analysis of the observer is given.