The hydrothermal methods have a wide application on crystal growth of single crystals and piezoelectric materials such as α -SiO 2, α -GeO 2, AlPO 4 and GaPO 4 may be synthesized by using this method. GaPO 4 is a very promising compound for piezoelectric devices because it exhibits both high coupling coefficient and high thermal stability (J. Crystal Growth 166 (1996) 347). However, good quality GaPO 4 single crystals is quite difficult to obtain due to the existence of two temperature ranges of solubility in acid aqueous solutions, formation of twins, inclusions of OH content in crystals and dislocations. Moreover, contrary to quartz, natural GaPO 4 crystals that can be used as seeds do not exist. GaPO 4 crystal growth investigation in both ranges of retrograde and direct solubility was realized at temperatures from 140 °C to 310 °C and pressures from 0.5 to 20 MPa by the hydrothermal method of temperature gradient. Aqueous solutions with additives of mineralizers from middle to very low pH (at ambient temperature and pressure) and GaPO 4 seed plates of different crystallographic orientations ( { 0 0 0 1 } , { 10 1 ¯ 0 } , { 10 1 ¯ 1 } , { 01 1 ¯ 1 } , { 10 1 ¯ 2 } , { 01 1 ¯ 2 } , { 11 2 ¯ 0 } , { 11 2 ¯ 1 } and others) were used. Crystals with weight up to 70 g were obtained. This study has allowed us to allocate several important parameters on stability of GaPO 4 phase, on growth rates of GaPO 4 crystals and GaPO 4 structural homogeneity. The characterization of GaPO 4 crystals has shown the better crystalline quality of crystals grown under conditions of direct solubility conditions. The strong influence of GaPO 4 solubility in aqueous acid solutions on crystalline quality either in retrograde or direct solubility ranges will be discussed in the present article.