The honor comes as no surprise since this is the 19th consecutive year the tycoon has topped the Forbes' billionaire list.
Berkshire Hathaway's Warren Buffett came in second with $46 billion. Oracle co-founder Larry Ellison is third with $41 billion. Brothers Charles and David Koch, powerful elite in the Republican Party, tied for fourth with $31 billion each.
Following the lackluster IPO of his company, Facebook founder Mark Zuckerberg dropped from 14 to 36 on the list.
According to Forbes, the list shows the rich mainly got richer this year, with rising stock prices and a rebound in real estate values.
The top 10 people on this year's Forbes 400 billionaires list are:
- Bill Gates, $66 billion
- Warren Buffett, $46 billion
- Larry Ellison, $41 billion
- Charles Koch, $31 billion
- David Koch, $31 billion
- Christy Walton & family, $27.9 billion
- Jim Walton, $26.8 billion
- Alice Walton, $26.3 billion
- S. Robson Walton, $26.1 billion
- Michael Bloomberg, $25 billion
Get more information from Forbes 2012 400 billionaires list.