Steam injectors can be used in many applications, but especially for security water injection in steam generators of nuclear reactors. Using a one-dimensional model, a steam injector with a centered liquid supply has been simulated. General relationships are presented from the nozzle exit to the steam injector outlet. It is shown that the flow contains a condensation shock. To achieve modelling of the mixing zone, and empirical correlation giving an equivalent pressure with value of condensation rate is found using experimental results obtained at the CETHIL. A parametric study is then made to determine the influence of significant parameters and the functioning range of the steam injector. Calculated values are compared with experimental results and are found to be in good agreement.