today i buy RB1200 But there is no fan i don’t know why 
Because it doesn’t have a fan as per its description, unless you buy an optional fan. Did you buy and pay for the optional fan?
http://routerboard.com/RB1200
RB1200
The new and affordable rackmount router.
It has ten individual gigabit Ethernet ports, five of them can be connected together in one 5-port switch group. RB1200 has a SODIMM slot with bundled 512MB of RAM, a beeper and a serial port.
It has no moving parts and it’s operation is completely silent, optional fan header is available. The RB1200 comes in a 1U aluminium rackmount case.
not having a fan is one of it’s benefits. many people didn’t like the noise of the RB1100, that’s why we made RB1200 without fan. It doesn’t get so hot.
Future RB1200 models will not even have the fan headers anymore.