Apple's stock has hit new highs recently because of optimism around what is believed to be the impending launch of the iPhone 5, and possibly a smaller, cheaper iPad.
Apple Inc. has been the world's most valuable company since the end of last year. It's now worth 53 per cent more than No. 2 Exxon Mobil Corp.
Apple's stock hit $664.74 in midday trading before retreating slightly to $663. That was $14.98, or 2.3 per cent, higher than Friday's close.
Microsoft's 1999 peak was $620.58 billion, according to Standard & Poor's.
The comparison to Microsoft does not take inflation into account. In inflation-adjusted dollars, the software giant was worth about $850 billion on Dec. 30, 1999. Microsoft is now worth $257 billion.
Analysts believe Apple's stock has room to grow. The average price target of 38 analysts polled by FactSet is $745.80.
Apart from the iPhone and "mini iPad," analysts are speculating that Apple plans to make a TV set to complete its suite of consumer electronics products. Apple usually doesn't comment on its future product plans until a few weeks or days before a launch.
China's largest oil company, PetroChina, was briefly worth $1 trillion after it listed on the Shanghai stock exchange in 2007, but only based on its price on that exchange. Its shares also trade in Hong Kong and on the New York Stock Exchange. Based on trading there, its market capitalization never went as high as $500 billion.
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