Abstract: Tobermorite is a kind of calcium silicate hydrate. By electron microprobe analysis, the chemical formula of tobermorite from Nammaw jadeite orebody in Myanmar is(Ca_4.424K_0.021Mg_0.003)_4.448 (Si_5.731Al_0.728)_6.465O₁₆(OH)₂·4H₂O, which is almost consistent with the classic chemical formula of Ca₅Si₆O₁₆(OH)·₂·4H₂O. The result of X-ray diffraction analysis(XRD) shows that the studied tobermorite belongs to the 1.4 nm type. Through microscopic observation, XRD and electron microprobe method, this tobermorite occurs as radial fibrous aggregate in yellowish-brown color, with low positive relief and I-order yellow interference color. The mineral is optically biaxial positive with an optic axial angle of 53°.