The study by the Triangle Center on Terrorism and Homeland Security said the number of Muslim-Americans who perpetrated or were arrested for terrorist acts fell from 47 in 2009 to 26 in 2010 and to 20 in 2011.
Charles Kurzman, sociology professor at the University of North Carolina Chapel Hill who wrote the study, noted even a single terrorist plot is too many.
"But this trend offers a challenge for the American public: If we ratchet up our security concerns when the rate of terrorism rises, should we ratchet down our concerns when it falls?" he said.
The study also found that the number of Muslim-Americans engaged in terrorist acts with domestic targets fell to 10 in 2010 from 18 in 2009.
The study said 75 percent of those engaged in alleged terrorist plots in 2010 were disrupted at an early stage of planning.
It said a large majority of the alleged Muslim-American terrorist activities (35 out of 46 individuals) occurred outside the United States.
The study said 11 Muslim-Americans have carried out terrorist attacks in the United States since Sept. 11, 2001, resulting in the deaths of 33 people.
The study said tips from the Muslim-American community helped disrupt terrorist plots in 48 of 120 cases involving Muslim-Americans.
"Is this a problem that deserves the attention of law enforcement and the Muslim-American community? Absolutely," said David Schanzer, the center's director. "But Americans should take note that these crimes are being perpetrated by a handful of people [whose] actions are denounced and rejected by virtually all the Muslims living in the United States."