In spite of the development of different experimental methods and theoretical models devoted to understand and predict the asphaltene behavior in dead/model oils, the asphaltene aggregation phenomena in live oil systems at high pressure-high temperature conditions has been grossly missing in the available literature. In this study, a population balance model with geometric scaling approach is proposed to simulate the asphaltene aggregates growth in time for live oil systems. Appropriate collision kernels are incorporated to describe the aggregation mechanisms taking into account the effect of pressure, temperature and oil characteristics. In modeling of the asphaltene aggregation phenomena in live oil, asphaltene aggregates are considered to be pressure-temperature dependent fractal structures. Data from depressurization experiments at different temperatures, performed by high pressure microscopy, are used to validate the model. Sensitivity analyses are performed to assess the effects of model parameters on simulation results. The experimental results which are obtained through the image analysis of the depressurization process in live oils are predicted by the model with a good accuracy. The fractal analysis of the asphaltene aggregates in live oil samples at elevated pressures and temperatures discloses that the fractal dimension of the asphaltene aggregates depends strongly on pressure and temperature of the system as well as on the nature of the oil. The proposed approach can well describe the prevailing mechanisms for asphaltene aggregation in live oils at pressure and temperature conditions of oilfields. (c) 2016 Elsevier B.V. All rights reserved.