The original script for this film was entitled "King Ghidorah's Great Counterattack" (Kingu Gidora no Daigyakushû), which featured Godzilla, Rodan and Varan fighting Ghidorah, Gigan, and a new monster named Mogu. It was then reworked into the more appealing "Godzilla Vs. The Space Monsters: The Earth Defense Command" (Gojira tai Uchû Kaijû: Chikyû Bôei Meirei). The script pitted Godzilla, Angilas, and Majin Tuol (a giant stone idol, similar to Daimajin) against King Ghidorah, Gigan, and Megalon, all three recruited by an evil alien brain called Miko. This second script was ultimately reworked into the film it is today. The Godzilla Tower, originally from the script, was part of a world fair amusement park, and was not made for an evil purpose like in the final film (in one scene, Gigan actually mistook it for the real Godzilla!). Megalon finally made his debut in Gojira tai Megaro (1973), and Majin Tuol was reworked into King Shisar in Gojira tai Mekagojira (1974).
This was Haruo Nakajima's final appearance as Godzilla before retiring from the role.
As a cost-saving measure, stock footage from San daikaijû: Chikyû saidai no kessen (1964), Kaijû daisensô (1965), Kaijû sôshingeki (1968) and other films were incorporated into the film (these films feature scenes of Godzilla, King Ghidorah and Angilas). Also, the entire score of the film was composed of re-used Akira Ifukube scores from numerous Toho Sci-Fi pieces.
This was the first film in which Godzilla actually drew blood. It first happens during the big battle later in the film when Gigan flies over past Godzilla and swipes his shoulder, and later, when Gigan repeatedly strikes a severely weakened Godzilla in his forehead.
Even though Ghidorah was "killed" in Kaijû sôshingeki (1968), his appearance in this film is explained by pointing out that the events of that film took place in the future date of 1999 while this film took place in the then present year of 1972.
This was the final appearance for King Ghidorah in a Showa era film. He later would return with a different look in Gojira vs. Kingu Gidorâ (1991).
For a long time it was shown on the movie's poster that Gigan could fire an eye beam from his forehead, above his eye, yet, this ability was never shown in the film. Many fans speculated that this was due to budget reasons, evident by the abundant amount of stock footage in the film. It was later revealed, however, by special effects director, Teruyoshi Nakano, that the real reason it was never shown was because he didn't believe it fits with the character. So, neither film demonstrated this ability for said reason. Additionally, Gigan also was said to possess the ability to breath fire, but that was never shown to in the film for the same reason as above.
The film received a limited theatrical release in the United States in 1978 by Cinema Shares as Godzilla on Monster Island.
Toho planned for this movie to be a return to form after Gojira tai Hedora (1971) baffled moviegoers and got its director banned from the series. This movie came about after attempts to make another Hedorah movie failed.
The device of gears under the clear dome in the World Children's Land Committee Office is made of Legos, painted gray.
The term, 'kaichou,' used for the youthful evil chairman is a pun in Japanese, which sounds very similar to 'kaiju', which translates as 'monster' and is the genre of the Gojira films.