I am just installing my new PC, after I assembly only motherboard,cpu and video card into chasses, I want to power on the pc to make sure everything is ok, but every time i power on or connect power cord, the power fan and cpu fan are just turning one second , then stop, I don't know what is going wrong? please help me, I appreciate it :(

Remove the motherboard from the case and try connecting it up like that. That will eliminate the possibility of any mounting screws shorting out the board.

thans for your answer, but how can I hold the motherboard while connecting it to power ? use my hand ?

I just Remove all the power connectors to Mb and short power supply Pins 14 and 15 coloured green and black, and switch on the power. I get same result: every time I power on, the power fan run second and stop I have no idea , help me!!!!

Some new motherboards use certain cpu fans which report their spinning speed, if the mobo does not receive the correct response it will shut down the pc. It might be worth checking your mobo manual, and processor fan

Two common things: #1: make sure your heatsink fan is plugged into the proper socket on the mobo; should say CPU FAN or FAN 1, or something like that. Most boards now have fan failsafe to protect the CPU from overheating and shut down the computer. If you are plugged into the wrong socket it the BIOS can't detect any RPMs so shut down the CPU. #2: Make sure you mounted the CPU properly, used thermal paste correctly and the heatsink is sitting properly on the mounting. It must sit very square and straight or it will overheat and shutdown in seconds.