The Tigers, who have hit nine homers in three CWS games and 102 overall, homered in three straight innings to break open a game that started 2½ hours late because of rain. LSU (54-16) has won 13 straight games.
The Tigers, playing for their sixth national title and first since 2000, await either Texas or Arizona State in the best-of-three finals beginning Monday. The Longhorns and Sun Devils play later tonight. Texas, 2-0 in the CWS, would wrap up its bracket with a victory. If Texas loses, the teams would play a deciding game Saturday.
Arkansas (41-24) was outscored 23-6 by LSU in two CWS games, and the Razorbacks lost four of five games overall against the Tigers this season. LSU shut out Arkansas for 14 straight innings over their two meetings in Omaha.
Ranaudo (11-3) allowed four hits and had five strikeouts against no walks. Austin Ross took over to start the seventh with the Tigers leading 11-0.
LSU's Blake Dean hit his second homer in three games, and 17th of the season, off Mike Bolsinger in the fifth.
Jared Mitchell, the Chicago White Sox's first-round draft pick, hit an opposite-field homer to left off former LSU pitcher TJ Forrest the next inning.
Ryan Schimpf followed with his team-leading 21st, sending Forrest's fastball near the top of the right-field stands and starting a five-run seventh.
Tyler Hanover added a pinch-hit two-run homer in the ninth off Zack Cox, the seventh of a CWS record-tying eight pitchers used by the Razorbacks.
Brett Eibner's two-run homer off Ross ended the Razorbacks' scoreless-inning streak in the seventh, and Chase Leavitt hit a three-run homer for Arkansas in the ninth.
Arkansas coach Dave Van Horn decided to start his closer, Stephen Richards, because No. 1 pitcher Dallas Keuchel had to work the last four innings in Arkansas' 4-3, 12-inning win over Virginia on Wednesday.
Richards (6-2), in his first start since April 2007, lasted two innings. He gave up four hits and four runs, two unearned.
Arkansas also was without shortstop Ben Tschepikow, who broke the ring finger on his left hand when he was hit by a pitch Wednesday. Cox, the regular third baseman, took over at short.