A close ally of Merkel’s and a one-time rival are considered favorites for the job to lead the center-right Christian Democratic Union.
Merkel announced in October she would give up the reins in her party, though she plans to serve her current term as chancellor.
Three high-profile contenders have toured Germany to drum up support.
Friday’s vote pits CDU general secretary Annegret Kramp-Karrenbauer, a Merkel ally, against Friedrich Merz, a former leader of the party’s parliamentary group who stands for a more conservative approach and has been away from front-line politics for a decade.
Health Minister Jens Spahn, another Merkel critic, is considered the outsider.