I'm using Mac connected to internet both of wired gigabit ethernet and 11n WiFi line. What line should my mac use? I want to force my Mac use gigabit ethernet primarily if possible.
Your Mac will use the highest-ranked active interface for its primary default route. All other active interfaces will allow connectivity to their local subnets.
Active means "link up". So on Ethernet, it's when you'd have a link light on the switch you're connected to. For AirPort it's when you're authenticated and associated to a wireless AP. For a modem or Bluetooth Dial Up Networking (DUN, using your mobile phone as a wireless modem), it's when you're dialed in and PPP is fully established.
Built-in Ethernet is ranked above AirPort by default. To view or change the rankings, go to System Preferences > Network and press the action (gear icon) button to get the action pop-down menu, then select "Set Service Order...". In the sheet that comes down, drag the interfaces names up and down in the list to reorder them.
It's usually safe to rank things like "Bluetooth DUN" above Ethernet or AirPort, because for most modern users, you'll almost never have a Bluetooth DUN connection active, and when you do, it's probably because it's the only network connection you've got at the time.