Hey guys, I have made my own arduino recently.
Want to ask something, in original UNO R3, when I powered it up using 9V battery, the LED at pin 13 will blink awhile right ?
However, for my own DIY board, it does not blink.
What I want to ask is that, why the LED at pin 13 will blink ? If never blink indicates what ?
FYI, I still can upload code from arduino IDE into my DDIY arduino board.
Thank you for help 
"FYI, I still can upload code from arduino IDE into my DDIY arduino board."
So what is your code doing with D13? If it does not set it up as an output pin and make it go high & low, then the LED will not bllink.
I mean in general when I power up my arduino, the LED connected to the pin 13 of arduino UNO will blink a while right then it only runs the code ?
It depends on what boot loader you have burnt into your home made system.
This is the wrong section. This is your project not an installation problem.
The bootloader blinks the pin13 (port B, bit 5; pin 10 on the actual DIP chip) when it first starts. The most likely causes for not seeing an LED blink on a homemade board are: 1) getting the pin numbers confused: pin13 is not pin13 of the chip. 2) LED not wired correctly (does it blink from a sketch?) 3) having a different bootloader
different bootloader means ? Normally in UNO is UNO bootloader ?
Yes. It would be unlikely to get some other bootloader, and you'd most likely be aware of it, but the bootloader does have "variables" for which pin to blink, so it's possible that you could get one that had been compiled by someone for a different board, with the LED in a different spot (or omitted.) (This would not necessarily break sketch uploading.)
(This is much more likely to show up on other cpus like the ATmega644 or ATmega1284, because those were never strongly standardized as to which pins were what.)