The aim of this research is to develop a semi-active secondary suspension system equipped with magnetorheological dampers to reduce the amplitude of vibrations and shocks to sensitive payloads in the cargo section of road truck. First, a double-ended magnetorheological damper was modeled, designed and built for use in a secondary suspension system of light trucks. Next, a pallet with a secondary suspension system consisting of four magnetoreheological dampers was constructed to be installed on the load side of the light truck. Then, the behavior of the system was examined by testing it by passing the vehicle on the speed-bump profile. By performing dynamic tests with harmonic excitation on the damper, the results showed that the maximum damping force with the electric current of 2 Amp is increased 11.6 times compared to 0 Amp. Furthermore by using the forces obtained from the Spencer model in predicting the dynamic behavior of the damper with an average relative error of %1.49 compared to the force obtained from the experimental test and implementing the two-dimensional model of the half-truck, the performance of the system in passing the speed-bump profile was investigated.The results of simulations and experimental tests showed that with increasing electric current intensity from 0 to 2 Amp, the maximum amplitudes of the sensitive payload decreased in passing the speed-bump %43.6 and %32.4 in simulations and experimental tests comparing to the situation without the secondary suspension system respectively.