The study researches vibational fatigue, more specifically, the means of determining the number of damaging cycles and their amplitudes through the life of a mechanical component. It tries to accomplish that in the time domain, working on time history of the loading, and also in frequency domain, based on power spectral density of random loading. Stochastic analysis of a time dependent process is described and ways of dealing with frequency domain methods are explained. A custom computer application is developed, which enables one to do numerical calculations and determine fatigue life based on given loading. A set of simulated loadings and a set of measured loadings are used to determine fatigue life using previously described methods. The frequency domain methods are then evaluated according to life estimates of the time domain method. One method in particular, developed by Dirlik, proves to be very reliable. As such it enables probabilistic approach to fatigue analysis and the same time reduces numerical complexity of the calculations.