hi, thx for your comments. I changed to be Listen 80. Now when I typed "wget http://myinstancefloatingip" from openstack host, I can see the downloaded index.html. But I cannot see Apache page on browser, meaning if I type "http://myinstancefloatingip" on my browsers (e.g. IE, firefox), I didn't see anything. There's no errors in log as well.

check yout firewall settings: inside the instance and also in openstack

do i need to modify iptables of both instance and openstack host? For instance, add NAT roles to translate fixed-ip to float-ip, or sth like it. Could you please tell me sth? I checked already, but still have no clue.

You need to open the ports on both: I would shutdown the firewall from the instance and also open all ports in openstack security. If it works after that, you can open only port 80/443.

Are you sure Apache is running ? You might want to check that as well.