He reached the post-season for the first time in his career, as the Phillies duplicated their 101-61 record from the previous season to face Sizemore's former team, the Dodgers, in the 1977 National League Championship Series. With the Phillies leading 5-3 in the ninth inning of game three, Sizemore committed an error on a throw from left fielder Greg Luzinski that opened the door for the Dodgers to score three runs and steal a 6-5 victory. It was Sizemore's second error of the series, won by the Dodgers in four games.