The Baltimore Sun, citing an anonymous source, reported that Mfume plans to make the announcement Tuesday.
According to the paper Mfume has said he would like time off to spend with his six sons, the youngest of whom is 14. With a schedule that includes 65 trips a year for the NAACP, he is also said to be seeking a break from the workload. Political sources also say he is considering a run for the Senate in 2006.
Calls to the NAACP, which is based in Baltimore, were not immediately returned late Monday night to The Associated Press. Mfume could not be reached for comment.
Mfume, 56, has been president of the National Association for the Advancement of Colored People since 1996.
Last month, the organization's chairman, Julian Bond, announced that its tax-exempt status is under review by the government in an investigation he contends stems from. Bond said IRS agents were investigating his keynote address July 11 at the NAACP's annual convention in Philadelphia.
For an organization to keep its tax-exempt status, "leaders cannot make partisan comments in official organization publications or at official organizational functions," according to an Oct. 8 letter to the NAACP from the IRS office in Louisville, Ky.
In September, the group launched a national billboard advertising campaign aimed at combating what officials describe as slowing membership growth. The civil rights group, founded in 1909, wants to increase membership by 20 percent, Mfume said at the time.
The group claims 500,000 members, but it has not seen significant membership growth in recent years.
Also in September, Mfume apologized to Education Secretary Rod Paige, whose invitation to speak at an Ohio NAACP dinner was withdrawn.
Mfume said he was "appalled" when he found out about the snub.
Mfume, who represented Baltimore's 7th district in the House of Representatives for nine years before taking over as head of the NAACP, inherited an organization tarnished by scandal and burdened by a $3.2 million debt when he took the helm.
The former talk-show host and radio station program director served on the Baltimore City Council from 1979-1987 before entering Congress and ascending to chairman of the Congressional Black Caucus. He served five terms before leaving for the NAACP in 1996.
Kweisi Mfume (pronounced Kwah-EE-see Oom-FOO-may) is a West African name meaning "conquering son of kings." He was born Frizzell Gray in Baltimore in 1948.