Main objective of thesis has been to discover the influence of ageing temperature and time on mechanical properties and microstructure of SINOXX 4542 precipitation hardening stainless steel. Theoretical part explains the basic principles of physical metallurgy of steel with emphasis on precipitation hardening kinetics in Fe-Cu binary phase system. The experimental part is divided into two parts. First part includes heat treatment (ageing) and mechanical testing of specimen, cut from hot rolled plate in solution annealed condition. American standard SAE AMS5604G prescribes 6 ageing conditions for 17-4 PH (SINOXX 4542) stainless steel and mechanical properties for each of them. Mechanical testing included tensile test, Charpy impact test, bending test, hardness test and hot tensile test. Measured mechanical properties were compared to prescribed values. Samples for each of 6 aged and solution treated conditions were metallographically examined with light and scanning electron microscope (SEM). Second part of experimental work was focused on sensitivity analysis for one of aged conditions (H900). Ageing temperature and time were variated. After heat treatment, mechanical tests were conducted. The results from testing were compared with values prescribed by American standard SAE AMS5604G. Results from light and scanning electron microscopy were analysed. The influences of microstructural changes on mechanical properties were studied. Final results showed that most of the measured mechanical properties were within the prescribed values, except yield strength Rp0,2 for aged condition H1150 and elongation A50 for H900. Results for sensitivity analysis of aged condition H900 showed that, we can age from 50 to 65 minutes at 482 °C, or 60 minutes at 490 °C and still reach prescribed values, but we didn’t reach them, when we aged our samples at 498 °C and 466 °C for 60 minutes and at 482 °C for 70 minutes.