how to get to Mutianyu from downtown beijing at August, 19.2012? is there any bus from Mutianyu to

There is buses for Mutianyu directly from Dongzhimen Long Distance Bus Terminal.
Everyday there are two buses leave for Mutianyu at 07:00 and 08:30 and go back to Beijing at 14:00 and 16:00. The whole journey will cost you about 12 RMB.

i confirm answer here, the bus number is 867, the stated bus fare is without yikatong (subway) card, if you will buy yikatong in any subway station the price is half and when you will be leaving beijing you can return yikatong and get rest of the money back, it's much more convenient to use yikatong while travelling in Beijing and it's also cheaper, bus fare is 0.4RMb compared to 1RMB, subway cost same and buses outside of Beijing cost half price, around 12RMB one way to Mutianyu while with yikatong it's 6.4RMB

in mutianyu parking lot there is black taxi mafia trying to cheat tourists there is no bus back and they try to hide information on bus stop about schedule, look for it in parking lot and no need to take taxi