A new upconversion nanocrystal phase Yb2Mo4O15:Er is developed by using a facile aqueous-precipitation procedure combined with thermal annealing. Nanocrystals of Yb2Mo4O15 are exclusively synthesized, with particle sizes ranging from 1 to 20 nm. The optical properties are characterized and a high upconversion quantum yield is determined to be approximate to 1.3% at room temperature, under excitation of approximate to 500 mW cm(-2) IR (975 nm). To the best of our knowledge, this is the first work concerning the synthesis of nanocrystalline Yb2Mo4O15 and the characterization of its upconversion properties, which possesses the potential to be utilized in bio-probing and thin-film optoelectronic device applications.