This is an MSc course dissertation project that aims to assess the effect of uncertainty to power system stability by using probabilistic study method. Small-signal stability is the only power system stability considered in this project and both Monte Carlo probabilistic method and Markov Chain Monte Carlo probabilistic method are applied. The objective for this project contains two parts. One is to study the influence of uncertainties to small-signal stability through Monte Carlo probabilistic method. The other one is to assess the benefit of Markov Chain Monte Carlo for this study compared with Monte Carlo method. The uncertainties considered in this project include load demand, wind power and solar power. Matlab is applied to model the uncertainties and to process the output data. All the power system studies are simulated under DigSILENT in a modified version of NET-NYPS test power system.