Mr Sanader had backed Ms Kosor, his former deputy, to replace him
Croatia's ruling party has elected prime minister-designate Jadranka Kosor as the party's new head after the surprise resignation of Ivo Sanader.
"I will lead both the government and the party with a strong female hand," Ms Kosor told delegates of the Croatian Democratic Union (HDZ) in Zagreb.
Parliament is expected to approve Ms Kosor as PM on Monday, making her Croatia's first woman leader.
Outgoing PM Sanader gave no clear reasons for his departure on Wednesday.
He was elected honorary party president at the HDZ convention.
Ms Kosor, who had been deputy prime minister since 2003, has been mandated to form a new centre-right coalition government.
Analysts say it is unlikely to differ greatly from Mr Sanader's team.
The 56-year-old journalist-turned-politician said her first task is to rebalance the budget this month, as Croatia battles a serious economic downturn.
Ms Kosor also pledged to resolve the border dispute with Slovenia and put Zagreb on a "fast track" towards its stalled EU accession bid.