Random Decrement Technique was developed for estimation of dynamic parameters when the inputs of structures are not known. The accuracy of estimation by the technique is investigated in this paper. For this purpose, a simulated structure is built and the estimated parameters by the technique are compared with the calculated values of the structure. The signals of accelerations and displacements of a simple beam are measured through a laboratory test. The signals are processed in time domain by Random Decrement Technique and in frequency domain. The estimated values of time-domain and frequency-domain are compared with each other and the results of comparing are analyzed. It is known from this paper that Random Decrement Technique is appropriate to estimate dynamic characteristics of civil engineering structures in random vibration when the modes are separated well.