At last, I can connect to AP using WPA after reading /usr/src/rt73-k2wrlz-1.1.0/IWPRIV_USAGE. On line 191, documentation is very clear:
d> Config STA to link with AP which is WPAPSK/TKIP(Authentication/Encryption)Now my remaining problem is to put those iwprivs into appropriate file in /etc/sysconfig/network-scripts/
1. iwpriv wlan0 set NetworkType=Infra
2. iwpriv wlan0 set AuthMode=WPAPSK
3. iwpriv wlan0 set EncrypType=TKIP
4. iwpriv wlan0 set SSID="AP's SSID"
5. iwpriv wlan0 set WPAPSK="12345678"
6. iwpriv wlan0 set SSID="AP's SSID"
p.s Step 4 is part of generating wpapsk password and is necessary.