A reliability study of nonlinear mechanical systems under random dynamic loads often requires Monte Carlo simulation in the time domain with hundreds of thousands of replications. The uncertainties involved in design make such analyses necessary for various admissible loads, which can be impractical. The authors have already developed a methodology that reduces the computational cost of Monte Carlo simulation when the load is represented by a Power Spectral Density (PSD) function. This method is based on a probabilistic re-analysis, which uses results from a simulation for a single PSD to estimate the reliability for other admissible PSDs. However, the methodology was limited to PSDs with the same energy content. This paper proposes an approach to extend the applicability of the above method to cases in which the energy content of the PSD functions changes.Keywords: Monte Carlo simulation; probabilistic re-analysis; random vibration; power spectral density function; probability of first-passage.Reference to this paper should be made as follows: Norouzi, M. and Nikolaidis, E. (2015) 'Probabilistic re-analysis of nonlinear systems when energy of excitation changes', Int. . Since 1980, he has studied reliability design of aircraft, turbines and ocean structures. His research focuses on the decision-based design of engineering systems when there is limited Probabilistic re-analysis of nonlinear systems 37 information. He has published three books and more than 100 papers. He is a fellow member of the ASME and chairs the quality, reliability and robust design committee of the Society of Automotive Engineers. This paper is a revised and expanded version of a paper entitled 'Efficient sensitivity analysis of reliability of nonlinear dynamic systems considering change in energy content of excitation' presented at the '