Mgnesio-sadanagaite is a new mineral species found in Japan in 1984. Its occurrences in the world are very scarce. It was first found by the authors in China in a ruby deposit in the Ailao Mountains, Yunnan Province. Its physical properties, chemical composition and X-ray powder diffraction and infrared absorption spectra have been studied. The finding of magnesio-sadana-gaite and its formation environment featuring AlzO3-rich, SiO2-poor, Cr2O3-bearing and FeO-poor indicated by its chemical compositions is of important theoretic and practical significance for the study of crystal chemistry of calcic amphiboles and for the exploration and evaluation of ruby deposits.