Abstract: The present article deals chiefly with the skillful utilization of gear trains in ancient China. The authors analyze three examples, all of which will be reconstructed for exhibition by the Historical Museum of China.The first example is the water-powered astronomical apparatus equipped with clock-work indicating the day of the month, which was invented by Chang Heng in the Han dynasty, about 130 A.D. The second example is the same kind of apparatus invented by I-Hsing and Liang Ling-tsan in the Tang dynasty, in the year 725 A.D. And the third example is the south-pointing carriage invented by Wu Te-jen in the Sung dynasty in the year 1107 A.D.From the above examples, we see that the ancient Chinese scientists already knew the following principles: l. by using a kind of gear train mechanism, that is, by the combination of a number of toothed wheels with different number of teeth, they could obtain a regular and uniform reduction of rotary motion; 2. By using gear trains they could transmit motion from one driver to two or more different followers with different speeds and directions; 3. By putting in or taking out an idler (or an intermediate wheel) between two toothed wheels, they could make the follower connect or disconnect with the driver; 4. By putting in one or several intermediate wheels between the driver and the follower, they could easily change the direction of rotation of the follower and reduce the total space needed by the whole train; 5. By putting in one intermediate wheel between two toothed wheels having the same number of teeth, they could get them to rotate with the same speed and direction. From all this, we can clearly see that, almost all the important principles relating to gear trains were discovered by our ancient scientists.