This study has examined the metallurgical microstructures and alloy compositions of 3 bronze artifacts, a vessel, a spoon and a chopstick, manufactured in the Koryo Dynasty. The results show that they were made from Cu-Sn alloys whose Sn content ranges from 22 to 24% by weight. It is of significant importance to find that they were all given special thermo-mechanical treatments during their manufacturing. It has been found that the Koryo bronze workers were well aware of the mechanical properties of ${alpha}$, ${eta}$, ${gamma}$ and ${delta}$ phases that appear in the Cu-Sn alloys. Knowing how to promote or suppress the formation of each phase, they were able to find proper alloy compositions and temperatures for the high Sn alloys to be successfully forged. The present study will detail the Koryo bronze technology as estimated from the microstructures of the 3 bronze artifacts. The results of the reproduction experiments with Cu-24%Sn will also be presented to explain how the microstructures develop and to estimate the mechanical properties of each phase.