A standing ovation from thousands of cheering fans coaxed two encores from singer Joni Mitchell at a concert last night in the LSU Assembly Center.

From the opening, "Help Me," through about 20 other songs, both her familiar hits and some new material, Miss Mitchell delighted her audience.

The L.A. Express, a smooth and pleasant five-piece band, warmed up the audience with a set and backed up Miss Mitchell on about half of her selections.

It was the selections on which Miss Mitchell accompanied herself on acoustic guitar of piano without the band that were best. This way, you could savor her rich, expressive voice with its full range and ruminate over the packed lyrics.

The Canadian folksinger, named best female vocalist last August in the first Rock Music Awards, loosened up with "I Was Raised on Robbery," during which she draped a gray feather boa around her shoulders and tucked a red rosebud from a fan over het right ear.

Just as at her March, 1974, concert at LSU, a vase of red roses was on the stage.

Among her selections were the vibrant "Free Man in Paris," the well-received "Big Yellow Tractor (sic)," and the familiar "Big Harry's House - Centerpiece" and "All I Want." A new selection was "Coyote and Don Juan's Wreckless (sic) Daughter." On another new selection, she placed the music on a stool in front of her.

After a concert of her clear and controlled voice, at time haunting and plaintive, then sprightly, Miss Mitchell trailed her boa and slipped off stage, returning with a lighted cigarette to sing two encores before waving to the audience with a decapitated rose stem and leaving her standing fans to find their way home.