"Only a baptized man (vir) validly receives sacred ordination." The Lord Jesus chose men (viri) to form the college of the twelve apostles, and the apostles did the same when they chose collaborators to succeed them in their ministry. The college of bishops, with whom the priests are united in the priesthood, makes the college of the twelve an ever-present and ever-active reality until Christ's return. The Church recognizes herself to be bound by this choice made by the Lord himself. For this reason, the ordination of women is not possible.
Source: Catechism of the Catholic Church (Vatican.va)