Serena Williams Serves Up $12 Million Bel-Air Mansion

Serena Williams is looking to double up on the massive Bel-Air mansion she bought for $6 mil in 2006 -- 'cause it's now on the market with a $12 mil price tag.

Unclear why Serena is unloading the property -- it's really cool, located in a really nice area and it's HUGE.

The home is 6,101 square-feet and looks like a palace -- with 6 beds, 7 baths -- and has a grassy area that spans roughly 2.7 acres and includes private hiking trails ('cause who's got time for L.A. traffic?).

The big surprise -- no tennis court!

So, where is Serena going? She's got multiple properties in Florida and her fiance Alexis Ohanian has a spot in NorCal ... so she's got options.