In this paper, the authors deal with a comparison of two different approaches (PERT method and Monte Carlo method) for calculation of the probability of a project completion. The PERT method is commonly used in a project management; the Monte Carlo is used less. The base assumption of authors can be expressed: the difference between the results obtained by the Monte Carlo method and PERT method is not significant with increasing number of simulations (iterations). For this reason, the hypothesis was formulated: there is no statistically significant difference between the calculated probabilities, i.e. both approaches are identical from application's point of view.