if one pci card shut off its power but still in slot, will the system still work well?

You're not talking about doing this just to save power are you? What types of cards are you interested in doing this with and why?

It depends on exactly what the card looks like electrically when it's power is shut off.

I've definitely seen cards that will prevent the system from booting if they aren't getting power, presumably because it prevented any communication on the entire PCI bus which -- on old systems with PCI-based videos cards -- was pretty much a non-starter.

If the card just looks like a Hi-Z load on the PCI bus when its power is off, it'll probably be fine.

You really need to look at the design of whatever chip it is that's hanging off the bus to figure this out. Note that many ICs have ESD protection diodes to both rails, so if the IC isn't getting any power, you're hanging diodes off the bus to ground, and this is likely to prevent the bus from working properly.

---Joel

well, the card concerned is just a nomal I/O card with a pci bridge linke PCI9054 from PLX tech yes ,just want to save power and full control of every card.

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thanks ,joel, I will check it. The bridge I use is PCI 9054 from PLX tech