Incommensurate deviation delta and quality factor Q of ZrTiO4 with the various cooling rates were measured. With decrease of cooling rate, the successive phase transitions were observed: high temperature. disordered normal phase (N) - incommensurately modulated ordered phase (IC) - commensurately ordered phase (C). It was ascribed to the IC - C transition that delta decreases and Q increases below the cooling rate of 1 degrees C/h.