An energy-based crack-growth model was developed in this study to simulate the propagation of top-down cracking in asphalt pavements. A viscoelastic fracture mechanics approach, the generalized J-integral, was employed to model the crack growth of asphalt concrete. Laboratory fatigue crack propagation tests for three asphalt mixtures were performed at various load levels, frequencies, and temperatures. Disk-shaped specimens with a proper loading fixture and crack growth monitoring system were selected for the tests. It was observed from the tests that the crack-propagation model based on the generalized J-integral was independent of load levels and frequencies, while the traditional Paris law model based on stress intensity factor was dependent on loading frequencies. However, both models were unable to deal with the temperature dependence of the mixtures. The fatigue crack propagation model proposed in this study had a good agreement between experimental and predicted crack-growth lives, an indicator that the energy-based J-integral could be a better parameter to describe fatigue crack propagation of viscoelastic materials such as asphalt mixtures.