Lee-Yang and Fisher zeros are crucial for study of phase transitions in the grand canonical and the canonical ensembles, respectively. However, these powerful methods do not cover the isothermalisobaric ensemble (NPT-ensemble), which reflects the conditions of many experiments. In this letter we present a new theory of the phase transitions in terms of the zeros of the NPT-ensemble partition functions in the complex plane. Proposed theory provides an approach to calculate all the partition function zeros in the NPT-ensemble, which form certain curves in the thermodynamic limit. To verify the theory we considered the Tonks gas and van der Waals fluid in the NPT-ensemble. In the case of Tonks gas, similarly to Lee-Yang circle theorem, we obtained an exact equation for the zeros limit curve. Also we derived the approximated limit curve equation for van der Waals fluid in terms of Szego curve. This curve fits numerically calculated zeros and correctly describes how the phenomenon of phase transition depends on the temperature.