Beyonce really disappointed by going casual to the Grammy Awards this year -- but last night, days after the event, she really stepped it up in a sexy and glamourous gown that showed off the right amount of cleavage.

The singer was sporting a stunning Elie Saab Couture dress to Tuesday night's premiere of her HBO special, "Beyonce: Life Is But a Dream" ... putting her decolletage on display at the party alongside hubby Jay-Z.

The bash was a family affair. While Blue Ivy stayed home with a nanny, Beyonce's mom Tina and sister Solange were both in the house -- and Solange even DJed!

And just to show you how big a deal the event was, Oprah Winfrey also made a rare red carpet appearance.

Check out the gallery above to see more shots from the party.

"Beyonce: Life Is But a Dream" airs this Saturday on HBO.

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