The effects of MnO2 addition on the piezoelectric properties in Pb(Mg1/3Nb2/3)O-3 relaxor ferroelectrics were studied in the ferroelectricity-dominated temperature range from -40 to 30degreesC. Dielectric, piezoelectric, and electrostrictive properties were examined to clarify the effect of MnO2 addition. As an added amount of MnO2 increases, the dielectric constant decreases and the mechanical quality factor increases in Pb(Mg1/3Nb2/3)O-3. From the experimental results, it has been found that Mn behaves as a ferroelectric domain pinning element.