The study, released Thursday by Transparency International UK, included companies from the 10 largest arms exporting countries such as the United States, Russia, Germany, France, the United Kingdom and China.
Those countries sell more than 90 percent of the weapons purchased around the world.
The global cost of corruption in the defense sector is at least $20 billion a year, Transparency International estimates.
The Defense Companies Anti-Corruption Index compiled by Transparency International analyses what 129 of the biggest defense companies in the world do and don't do to prevent corruption. The companies received grades of A to F.
The companies rated are worth more than $10 trillion and have combined defense revenue of more than $500 billion.
Some 85 percent of defense company leaders don't speak enough publicly about the importance of preventing corruption, the report said. CEOs who "actively promote a values culture" have a greater effect on reducing corrupt activities, TI said.
The defense industry appears to be improving its attitudes toward corruption, said Mark Pyman, director of Transparency International UK's Defense and Security Program.
Ten percent of companies now have good disclosure of their anti-corruption practices, which Pyman said was "much more than it would have been 10 years ago."