The organization Roman Catholic Womenpriests says it's ordaining Lillian Lewis, 75, today.
Bishop Joan Hauk, who was ordained through the group, will officiate, said spokeswoman the Rev. Suzanne Thiel.
Womenpriests began ordaining women in Germany in 2002 and now has 182 priests and priesthood candidates worldwide, 146 of them in the U.S., Thiel told The Associated Press.
Bishop Paul Bradley, head of the Roman Catholic Diocese of Kalamazoo, issued a statement Wednesday sharply critical of the ordination.
He said that Womenpriests "is not recognized within the universal or the local Catholic Church. Please be aware that such an action does not confer the sacred character of the priesthood for this woman."
"If this invalid 'ordination' takes place then the woman attempting ordination incurs an automatic excommunication," Bradley said. "Any Catholic attending or participating in this, or any invalid and illicit attempt at the sacrament of ordination, places themselves outside of full communion with the Catholic Church."
Thiel said that women ordained through her group have been successful in building religious communities.
"We ... have over 70 communities around the country where people have received women as priests," she said. "The communities where women are officiating are flourishing."
Lewis, a Chicago native and longtime worker in Catholic congregations, told the Kalamazoo Gazette she knows she faces excommunication but feels strongly that women should be priests.
"I don't work for the church anymore. My real job now is (to) give witness and stand up for what I believe in," she said.
The service was originally going to be held at First Congregational Church in Three Oaks. But Lewis told the newspaper that it will be held at her home instead because the church has received harassing phone calls.