The “end of active transmission” will be declared, after 42 days without a new case in Liberia.
It joins Guinea and Sierra Leone, which earned the status last year.
However, UN chief Ban Ki-moon has warned that West Africa may see flare-ups of the virus. It has killed more than 11,000 people since December 2013.
‘Most critical’ months
A country is considered free of human-to-human transmission once two 21-day incubation periods have passed since the last known case tested negative for a second time.
However, the end of active transmission of Ebola has been declared twice before in Liberia – only for the infection to re-emerge.
This is why the expected declaration by WHO later on Thursday will be marked with caution, BBC Africa’s health correspondent Anne Soy says.
On Wednesday, Mr Ban warned that “we can anticipate future flare-ups of Ebola in the coming year”.
“But we also expect the potential and frequency of those flare-ups to decrease over time,” he added.
Meanwhile, WHO chief Margaret Chan said the virus could persist in some Ebola survivors even after their full recovery.
“By the end of this year, we expect that all survivors will have cleared the virus from their bodies,” Ms Chan was quoted as saying by the AFP news agency.
She also described the next three months as “the most critical” for the three West African nations.