Measurement standard for AC RMS voltage has been realized, by using thermal converters as AC-DC transfer standards in every country. AC-DC transfer standards provide a link for AC RMS voltage to DC voltage. Since thermal converters are physical artefacts, they have characteristic error defined by AC-DC difference. By adopting MEMS technology, a high performance thermal converter, called planar multi-junction thermal converter (PMJTC), with low AC-DC transfer difference and high output voltage has been successfully developed, and widely used as primary AC-DC transfer standard in many countries. In the Republic of Korea, AC-DC transfer standards 10 Hz to 1 MHz in frequency for 2 mV_rms to 1000 V_rms have been established by using PMJTC.