We developed a facile hydrothermal method for synthesizing ultrafine size-controllable earth-abundant Cu 2 ZnSnS 4 (CZTS) nanocrystals using simple Cu(II), Zn(II) and Sn(II) inorganic salts and thiourea in a mixed ethylenediamine and di-water solution as precursors. X-ray diffraction, Raman scattering and transmission electron microscopy confirm that pure kesterite structure CZTS nanocrystals have been synthesized at temperatures as low as 180 C. Broadening of Raman peaks and blue-shift of the absorption edge is attributed to quantum confinement within the nanocrystals. The hydrophilism and tunable band-gap of the CZTS nanocrystals show the potential applications of the nanocrystals for biological labelling and quantum dot based solar cells.