An X-ray study of the effect of the pressure on the incommensurate phase transition in ZrTiO4 was performed in order to obtain the information concerning the nature of the competing forces that give rise to a structural instability at a general wave-vector. It was found that an increase in pressure lowers the transition temperature (dT(i)/dP(i) congruent to -6.9 K/kbar). The pressure dependence of the incommensurate to commensurate transition was also investigated (dT(c)/dP(c) congruent to -9.0 K/kbar).