A dual-channel automatic ac-dc voltage transfer error measurement system in which the output voltages of two thermal voltage converters which are ac voltage standard are directly measured at the same time to reduce the output voltage drift is described. Forward-reverse measurement method by using a two-channel scanner is used to cancel the offset voltage of the voltmeters. The agreements of the 4-V TVC comparison results between other national standards institute and Korea Research Institute of Standards and Science were less than about ${pm}2;ppm$ in the frequency range of $40;Hz{sim}100;kHz$, and were less than about ${pm}4;ppm$ at $200;kHz{sim}1;MHz$. Measurement uncertainty is reduced significantly from ${pm}4;ppm$ of manual system to ${pm}3;ppm$ of new system(up to 100 kHz) typically and great increase in comparison efficiency has been achieved by this system.