One of the missions of the INERIS is to assess accidental risks induced by industrial activities. The use of liquid gases is very widespread in industry but a large potential hazard exists in case of accidental release. These substances can be flammable and explosive like the LPG or toxic like anhydrous ammonia. In case of an accidental leakage due to a vessel breach or a line rupture, a violently change phase could take place inside theses products. This phenomenon called flashing jet are still bad understood and many strong hypothesis are often used to predict by modelling its physical consequences in the near field like thermodynamic behaviour of the whole jet, pool formation. However all these physical consequences have to be well evaluated in order to better estimate the explosive cloud or toxic cloud formation. The objective of the INERIS-CORIA work is to develop a new numerical model with the aim of simulating two phase jet resulting from a leakage in a pipe containing a liquefied gas. The work focuses especially on the effect of the vaporization and boiling process in the jet. A thermodynamic equilibrium model for vaporization was thus developed. To test the model, an atmospheric two-phase jet of butane, emanating from a circular orifice is considered. The modelling results show that the calculated temperature behaviour in the spray jet by comparison with the observations is generally satisfactory. This result cannot be obtained with classical vaporization model.