During the 90s the Short- Distance Maritime Transport grew by 29.6% and it keeps growing, since it has a much higher potential. Several public administrations, like the European Union and Member States, are pursuing to promote Short Sea Shipping. Nowadays, most of the SSS lines work by using roll-on/roll-off cargo vessels, particulary the ones transporting mixed cargo (trucks and semitrailers) and passengers, known as Ro-PAx. One of the most relevant aspects of maritime transport instead of road transport are unitary costs, competitive due to economies of scale. The aim of this article is to study how do economies of scale develop in Ro-PAx vessels based on the cost modeling affecting the shipping companies. To achieve this aim an empirical analysis of Ro-Pax vessels currently operating in Europe will be developed, analysing the most influentia variables on explotation costs. Literature has mainly focused on containerships and bulkcarriers so far.