Morales, who took office in 2006 as the country's first indigenous president, finished nearly 40 points ahead of his nearest rival across the country, found pollsters Ipsos and Equipos Mori.
They found the runner-up was wealthy cement magnate Samuel Doria Medina with around 20 percent of the vote.
Morales's Movement Toward Socialism (MAS) is also expected to win a strong majority in Congress to continue implementing leftist reforms that have so far included nationalizing the oil and gas sectors and expanding welfare programs in one of Latin America's poorest countries.
Partial official results with about 70 percent of all ballots counted were expected to be released around midnight, according to the Supreme Electoral Tribunal.
If confirmed, Morales will extend his time in office to 14 years, until January 2020, after Bolivia's Supreme Court ruled last year that his first term was exempt from a new constitution adopted in 2009 that imposed a limit of one re-election for sitting presidents.
Morales, who rose to prominence as a union leader fighting for the rights of the country's coca growers, has brought sweeping changes since taking office in 2006.
His government has nationalized the oil and gas sectors and expanded welfare programs.
The economy grew 6.8 percent last year and is forecast to grow more than five percent this year, one of the fastest rates in Latin America.
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