The singer says she's sent out letters of intent to sue to the rapper, Murder Inc Records and Def Jam for money she claims she's owed for her part in the 2001 hit song PUT IT ON ME.
She tells Allhiphop.com, "That (song) helped boost (Ja Rule's) career. That was a huge song. I helped make him an icon - so they have to pay.
"I had to search for the right lawyer that would take the case so they wouldn't have any ties to Murder Inc or Def Jam or anything to do with that song. I told the lawyer, 'It's not about the money, it's about the principle.'
"Technically, off of that I could have retired or I could have gotten a crazy publishing deal."
Lil' Mo, real name CYNTHIA LOVING, claims that she owned 10 per cent of the song's publishing, and tried her best to contact those who owed her the money before she sought legal advice.
She adds, "This is beyond personal, this is business. This is my livelihood as an artist. We don't make money until the label makes money so as a songwriter that was a part of my career that I was supposed to receive a substantial amount of money for."
Representatives for Murder Inc and Def Jam have yet to comment on the suit.