The effect of Mn doping on the dielectric properties of Ba2Ti9O20 ceramics has been investigated at microwave frequency. The dielectric constant and the temperature coefficient are not sensitive to the addition of Mn, while the unloaded Q depends strongly upon it as well as on heat treatment at high temperature. A minute addition of Mn leads to an increase in the Q value. A high value of Q of more than 5200 was obtained at 9 GHz with the addition of 0.5∼1.0 mol% Mn.