The precipitation-hardening steels are generally required when high mechanical performances at high temperatures are requested both statically and dynamically. The requested combination of mechanical strength and toughness is achieved by means of an age-hardening treatment which can be carried out at different temperatures and times. Aging 17-4 PH (precipitation-hardening) in the temperature range of 480–620°C results in a precipitation hardening due to the formation of a submicroscopic, copper-rich phase. In the technical literature, the investigation of the phase transformation and the age-hardening behavior of the steel has been carried out at different stages of aging by different techniques, such as transmission electron microscopy studies and x-ray diffraction of the precipitates together with optical and scanning electron microscope analysis of the microstructure. In this paper an alternative method based on dilatometric experiments is proposed to study the kinetics of the age hardening of the steel. The experiments have been carried out at different temperatures on quenched samples and the transformation of the structure during aging has been continuously followed up to 8 h. For a better investigation of the data obtained from the dilatometric analysis, mechanical tests, notch impact tests, and microstructural analysis have been carried out at different and selected parameters of the aging.