A bus shock absorber of double-tube type was tested with rubber bushings installed on a dynamic testing system using electrohydraulic servo principle. The transmitted force and displacement were measured at different frequencies up to 10 Hz. Test results showed the frequency dependency of quite a large hysteresis loop in force vs. velocity diagrams and significant asymmetrical damping in extension and compression. A five-parameter shock absorber model was developed based on a linear valve characteristic assumption. The influence of the rubber bushing was analyzed and considered as a contribution to the effective compressibility of the system. The Gauss-Newton method was utilized to estimate the model parameters by time-domain nonlinear curve fitting. The model was in good agreement with the test data, specially in the extension stage and in the higher frequency range. The estimated parameters were consistent with the physical configuration of the shock absorber.