The triode vacuum tube (so-called "Audion") was invented in 1906 by Lee de Forest (1873-1961), an American inventor-scientist. It was the most important electronic device until the transister was developed in 1948. Most Historians of technology who had investgated the origin of triode vacuum tube argued that de Forest's invention resulted from the modification of the Fleming valve. In this thesis, I argue that the invention of the triode vacuum tube originated from de Forest's independent research. Since his graduation from Yale Shefield Scientific School in 1899, de Forest had worked on the detectors for wireless communition. He succeeded in developing the electrolytic detector in 1903 and the flame detector in 1905, which consolidated his belief that the control of ions would play an important role in these devices. His grid "Audion" in 1906 was the fruit of this line of research, and it made his conviction even stronger that ionized gases were the main charge carriers in Audion. In this respect, his understanding of vacuum tube, whether it was Audion's or Fleming's valve, was quite different from Fleming's; in the latter case, the "electrons" rather than "ions" were the charge carriers. In conclusion, the triode vacuum tube originated not from the modification of the Fleming valve but from de Forest's original research on detectors. In the whole process of invention and modification, the most important factor was de Forest's belief on the role of ions in tube. However, it prevented him from improving the Audion to higher vacuum tube, which would be carried out by the new generation of engineer-scientists.