Musharraf ousted Sharif in a military coup eight years ago.
Speaking to reporters in London, Sharif dismissed fears that Musharraf could have him arrested upon arrival, Voice of America said Friday.
"He kept me 14 months in jail," said Sharif. "He now once again is threatening me. I'm not scared …"
Sharif is demanding Musharraf step down as president and head of Pakistan's military.
Musharraf is reportedly seeking the endorsement of another former prime minister, Benazir Bhutto, in an effort to bolster public support for his re-election.
Sharif's return intensifies the pressure on both Musharraf and Bhutto, who lives in self-imposed exile but still heads Pakistan's largest opposition political party, to resolve their differences.
Bhutto is demanding the government drop corruption charges against her and her family and end restrictions preventing her from serving a third term as prime minister.
She also insists Musharraf resign as commander of the army.
Sharif has criticized the talks and vowed he wouldn't cut a deal with Musharraf.