Hydrothermal synthesis of zinc oxide crystals in supercritical formic acid (FA) solutions was carried out at 400degreesC. Increasing FA concentration from 0.0 to 1.0 mol.kg(-1) gave hexagonal plates, hexagonal pillars, and tube-like shapes. Depending on FA concentration, relative intensity of emission spectra at around 500 nm increased from 0.53 to 1.0, peak at 388 nm shifted to 393 nm, and atomic ratio O/(O + Zn) decreased from 49 to 38%.