Thermal expansion of manganosite MnO at temperatures between 20°C and 850°C has been carefully determined by a dilatometric technique. The theory of thermal expansion by Gruneisen is improved and the acquired data are analyzed to derive the harmonic and anharmonic parameters of MnO. Gruneisen's parameter γ is 1.56, Debye temperature is 441 K, and pressure derivative of bulk modulus is 7. The volume expansion coefficient is 34.3×10^-6/K at 20°C and 47.1×10^-6/K at 850°C; almost the same in magnitude to those of MgO and FeO.