The establishment of the factors influencing the number of package shipments, the elaboration of predictive models for the number of package shipments and the definition of the methodology for the necessary capacity of means of transport are important for the efficient planning of transport capacities, because the number of package shipments on the postal services market keeps increasing each year. Throughout its chapters, this doctoral thesis defines the determinants of the model of the influence of factors on the number of package shipments, explains the correlation and regression analyses and analyses the factors that influence the number of package shipments. The above is followed by a description of the elaboration procedures of regression models, which have subsequently been tested. In their elaboration, the factors that influence the number of package shipments have been evaluated. On the basis of previous research and research conducted in one postal service provider in the Republic of Croatia, the transport efficiency indicators were selected, the average mass and volume of package shipments calculated and the methodology for establishing the necessary capacity of means of transport defined, in order to improve the efficiency of package shipments in postal traffic. On the basis of correlation analysis, the factors that influence the number of package shipments have been evaluated and variables and factors for the elaboration of the predictive model selected. By means of the regression analysis, the selected variables, on the basis of which two predictive models have been elaborated, were also evaluated. The models were tested on the basis of real data, which helped verify their applicability in the prediction of package shipments in future periods. The results of the predictive model for the number of package shipments were also compared to the planned number of package shipments in the observed postal service provider, which additionally confirmed the applicability of the predictive model. Apart from the selected efficiency indicators, the methodology for the establishment of the necessary capacities of means of transport between the entities of the postal network was also defined and applied. The evaluation of the factors influencing the number of package shipments, application of the model for predicting the number of package shipments and use of the defined methodology for the establishment of the necessary capacities of means of transport and the improvement of package transport efficiency in postal traffic offer a possibility for optimisation of the vehicle fleet.